GtkPopover: avoid a critical

focus_widget can be NULL, as pointed out in
https://bugzilla.gnome.org/show_bug.cgi?id=723181#c7
docs: Improve grammar in GtkHeaderBar overview
Ignore hide-titlebar-when-maximized for CSD

This setting really only makes sense for wm titlebars,
client-side titlebars are under application control
anyway.
gdkwindow: Use an InputOnly window for focus_window

For XWayland, this is somewhat important to prevent a dummy XWayland surface
being made for all clients that connect...

https://bugzilla.gnome.org/show_bug.cgi?id=723390
gdkwindow-x11: Fix compile warning

It doesn't actually matter, since we don't pass any real attributes
anyway, but this should be the correct type...
gtkicontheme: Don't query CONTENT_TYPE to determine if something is an SVG

We already know based on the suffix of the filename.

https://bugzilla.gnome.org/show_bug.cgi?id=721895
Update ui.h file for GtkFileChooserDefault rename
gtk3-demo: Fix two issues with the builder demo

The ui file was generating a warning about AtkAction not
being implemented, and the about dialog had a missing icon.
gdkwindow-wayland: Bail out early if we get a frame callback when destroying our window

This prevents warnings like

(gtk3-demo:14948): Gdk-CRITICAL **: _gdk_frame_clock_thaw: assertion 'GDK_IS_FRAME_CLOCK (clock)' failed

(gtk3-demo:14948): Gdk-CRITICAL **: gdk_frame_clock_get_timings: assertion 'GDK_IS_FRAME_CLOCK (frame_clock)' failed

We need to do this, as the compositor might have already sent us a frame
event, in-flight, at the same time we destroy our window. In this case, we'll
receive the then-in-flight "done" event, and then warn as we try to look
up the frame clock on a destroyed window.
gdkwindow-wayland: Erm, put the DESTROYED check in the right spot...
Updated POTFILES.in
Updated Traditional Chinese translation(Hong Kong and Taiwan)
Fix a case of nested comments
Updated Greek translation
docs: use markdown instead of docbook tags
docs: list items require blank lines
docs: remove spurious para
Updated Indonesian translation
Updated Kazakh translation
Updated POTFILES.in and POTFILES.skip