- all changes now in wip/jimmac/gnome-3-32 branch

a11y: Check X11 display at runtime

See merge request !450

`gtk_widget_accessible_grab_focus()` code checks that X11 isenabled at
build time and uses X11 specific functions such as
`gdk_x11_get_server_time()` regardless of the actual backend being used.

Check that we are using an X11 display when X11 is backend enabled, so
we do not crash when running on Wayland

Closes: #1507

- switches in particular looked undefined

- defining gradients outside of _drawing while Lapo isn't
  looking. Surely this will bite us in the future.

- based on a patch by frederik.feichtmeier <frederik.feichtmeier@gmail.com>
  I'm certain this is something we had initially, but can't recall
  why we got rid of it for the more visually distracting dashed line.
  We can always revert when Lapo shows up and slams us with that broken
  use case. I'm guessing non-white bgs.

- So far it looks way less distracting than the dashed line

GDK W32: Always set gtk-font-name to the active UI font. Fixes #1484

See merge request !436

Changing FIXME on unneeded functions to X11-only where appropriate.

According to Apple's docs it's no longer necessary as Cocoa will notice
any changes and call drawRect as needed.

It was deprecated in 10.8 but no replacement until 10.10.

Runtime check alone doesn't silence warning.

Handling more flags, handling them correctly, and emitting the requisite
signals.

Change screen layout to use CGGetActiveDisplayList instead of NSScreens,
eliminating the latency between updating screens and recomputing the
root window.

Moving the initialization of the GdkQuartzMonitors to GdkQuartzDisplay from
the now-obsolete GdkQuartzScreen. Use QuartzDisplayServices for
monitor enumeration and to populate the GdkMonitor properties. This is
better aligned with acting on the Quartz Services callbacks for monitor
changes and with Cairo which also uses CoreGraphics for drawing.

gtk_internal_return_val_if_fail operates only in debug mode,
quartz can call this with a NULL that crashes in
GTK_STYLE_PROVIDER_PRIVATE_GET_INTERFACE.

With convertPointToScreen & convertPointFromScreen when
MacOS version is > =10.7, when the earlier functions were
deprecated and replaced.

Makes auditing easier.

- less dramatic gradient on the buttons

- for the previous patch

GtkTheme: Menu border-radius and box-shadow changes

See merge request !439

This makes apps use "Segoe UI 9" by default instead of whatever matches "Sans 10".
It also cleans up the code and uses some new pango API while at it.

This was previously disabled in 9e686d1f because it led to a poor glyph coverage
on certain versions of Windows which don't default to "Segoe UI 9" (Chinese, Korean, ..)
because the font fallback list was missing in pango.

This is about to get fixed in pango!34
so enable it again when we detect a new enough pango version.

- to mimic what happened to regular borders

- introduce $menu_radius
- use it for menus and context-menus
- use the popover box-shadow also for menus
- use padding for menus to avoid edge overlapping
- remove the background for menus to avoid bleeding out of the round edges

- tone down the button z-depth
- flatter headerbars