GitLab

It is fine to open an empty .ui file, since we will write the proper
<interface/> element and such when saving it back to disk.

Fixes #682

These are set automatically, according to

  https://github.com/flathub/org.gnome.Builder/pull/6/files

When guessing the language from GtkSourceView, we can remove the trailing
.in so that we are more likely to get the proper language type. This is
useful because rarely are people simply caring about the template expansion
and more interested in the underlying language syntax.

Fixes #681

I wanted to avoid this originally, since .ui files can also be used with
other systems such as Qt, but we are a GNOME focused IDE and there is very
little we could ever do to support Qt, so just mark the .ui glob as being
a gtk-builder file.

This simplifies our signal tracking, and allows us a convenient place to
start doing tracking of more GladeProject signal editing.

When saving the designer view, we want to reload the editor source for the
user so that they don't need to get a "modified on disk" warning. This
requires a bit more manual state tracking, but it's worth it.

This allows us to reload a glade project in what seems like "in-place".
Supporting this will allow us to track changes from the editor immediately.

This makes the visibility more similar to what we do with the symbol
popover, which is the closest metaphor here.

These are slightly more useful than the previous tooltip text.

This adds actions for undo/redo and wires up shortcuts to them. It also
adds menu entries for the document menu.

Glade doesn't generate 2x icons, so we probably want to make something new
for these values.

This means that we don't double up on bars, and that is probably a good
thing for hit targets and what not.

I'm not sure the lifetime of the glade plugin, because we know that
we'll have to write something new for gtk4. But we have a couple of more
releases with gtk3, so we can use this for now.

There is still much more to do, but this is basic enough that it works
to a degree that is useful.

Some of the missing parts include:

 * Better visibility of what is going on
 * Toolbar for various palette widgets
 * Hooking up signals to source code
 * Missing icons which are in the works

and probably a bunch more.

This allows specifying a workbench-addin that can open the file by
providing the hint matching the addins identifier.

This uses the dockbin to more properly dismiss the panel so that we are
unpinned before dismissal. Doing so means we do less drawing and therefore
reach a higher frame-rate.

We already do the same thing with the IdeEditorSidebar for the same reason.

This allows us to keep the sidebar open for longer when we know we don't
want it to disappear.