gdk: seal in-memory files when possible

See merge request GNOME/gtk!353

GtkBuildable: Fix the type of the user_data in GtkBuildable.custom_tag_end

See merge request GNOME/gtk!378

Commit 64a489ad inadvertently introduced a regression that broke Korean
text input because the changes there resulted that only the last input
string that we have from ImmGetCompositionStringW() for each time the
commit signal is emitted is kept, and also as a result the final Korean
character that is input by hitting space is also lost as a result, as we
didn't check for whether we are done with preediting.

Fix these issues by doing the following when we receive the
WM_IME_COMPOSITION message with GCS_RESULTSTR from Windows:
-Do not emit the commit signal during WM_IME_ENDCOMPOSITION, and...
-Emit the commit signal anyways, as we did before c255ba68, however...
-We still save up the string to commit, because we need to re-compute
 the cursor position when we do ->get_preedit_string(), which needs to
 take the GCS_RESULTSTR string we get from WM_IME_COMPOSITION into
 account as well, so that we avoid getting the Pango criticals that
 occur during Chinese (and most likely Japanese) input as the cursor
 position is out-of-range.

Fixes issue #1350.

Pointer arithmetic on gpointers (void *) is a GCCism, so avoid that by
being more explicit with things.

Visual Studio is unlikely to support VLAs at any point, so avoid using
them and use g_newa() instead.

The previous type was a pointer to a pointer, which seems to be a copy-paste
error from GtkBuildable.custom_tag_start which is an out parameter. It was
always cast in use so this is an API break, but not an ABI one.

gtkstack: fix null pointer dereference

See merge request GNOME/gtk!361

GDK W32: Support switching IM contexts at runtime (GTK4)

See merge request GNOME/gtk!372

This leverages the normal input context switching mechanism in GTK
by making it think that the gtk-im-module setting changed.
The backend returns gtk-im-module value as "ime" if W32
IME API says that an IME is in use. Otherwise it returns
and empty string - this still triggers an input context
switching code, which, not being able to create the desired context
(which is and empty string), falls back to looking at current
keyboard layout (currently that code is still a FIXME).

Paired with the code that signals gtk-im-module change on keyboard layout
switches, this is sufficient to make GTK capable of switching to
the appropriate IM context at runtime. At least, the kinds of context
that specify languages for which they are used automatically by default
(once locale matching is implemented), and the IME context.

Loading other kinds of IM context might still work via specifying
the gtk-im-module setting in gtk ini file, but doing so will likely
make GTK incapable of using the IME context that is used
for Korean, Chinese and Japanese (and some other languages).

Until someone figures out a way to actually change gtk-im-module
setting on Windows at runtime with meaningful values, the behaviour
introduced by this commit seems like a sufficient workaround.

Fixes #1383

The widget observer APIs belong into the widget docs, not the window
docs. Oops.

- don't put the gradient/cogged wheel on apps that have a split headerbar
  due to having a sidebar

This can be used by compositors to mmap memory without having to
handle SIGBUS.

The gtk_stack_snapshot_slide() function dereferences the
last_visible_child pointer without proper != NULL ckeck. This might
result in NULL pointer dereference and crash if last_visible_child is
invalid.

Add a != NULL check before dereferencing the pointer.

docs: Fix inclusion of source files in tutorial

See merge request GNOME/gtk!341

Adwaita: tone down the 'devel' styling

See merge request GNOME/gtk!357

The selected bg was too prominent and intefered with button styling.

Make dashed border-style work correctly

See merge request GNOME/gtk!355

We don't technically hide the arrow nodes anymore.

Arnaud B. authored Sep 21, 2018 and Arnaud B. committed Sep 24, 2018

There’s a short-path done for focus rectangles, but it can be taken in other conditions, and then fail occasionally to render a dashed line if the border-width is too big.

Unneeded now.

Jan Alexander Steffens authored Sep 05, 2018 and Daniel Boles committed Sep 22, 2018

Close GNOME/gtk#1323 (comment 327705)

flattenlistmodel: Fix uninitialized variable warning

See merge request GNOME/gtk!347

See https://developer.gnome.org/hig/stable/typography.html

https://bugzilla.gnome.org/show_bug.cgi?id=772371

Variable, added, would be a garbage value if model is NULL and
the following code, if condition, use the uninitialized variable.
A side effect could be occurred by that.

To avoid, the variable is initialized to zero.

Remove some duplication in testfilechooserbutton

See merge request GNOME/gtk!342

Get GTK+ 4 installed-tests closer to working

See merge request GNOME/gtk!348