wayland: don't init if XDG_RUNTIME_DIR missing

wayland doesn't strictly follow the XDG_RUNTIME_DIR spec by falling back
to another directory in case the runtime dir is not properly set.

When this variable is unset, wayland will log an error to us, which we
treat as fatal, aborting the entire program.

Skip ourselves all the trouble and don't try to bring up the wayland
backend when we know it will fail in this way.

https://bugzilla.gnome.org/show_bug.cgi?id=738873
gdk/broadway/toarray.pl: Fix Code Generation for MSVC

The current implementation of this script generate headers with \x-escaped
strings that can become too long (> 65535 characters) for Visual Studio
to consume, hence the build of broadwayd would break on Visual Studio.

This changes the script to instead format the string as an array of hex
characters, not unlike what GResource does, so that builds can continue as
normal on Visual Studio builds as well.

https://bugzilla.gnome.org/show_bug.cgi?id=739001
Fix Up Visual Studio Property Sheets

"Install" gdkbroadwaydisplay.h, as it is a public header of GDK Broadway,
also fix the MSVC 2010 gtk-copy-gdk-broadway.props property sheet as a
'>' is missing.  Sorry for not noting these issues earlier, as MSVC
Broadway builds were recently restored.  My bad :|
window: Update popover positions on ::size-allocate

Popovers may get relocations optimized away if only x/y changed
in the GtkAllocation. So make sure the toplevel updates popover
positions on all situations.

https://bugzilla.gnome.org/show_bug.cgi?id=729140
Don't forget to allocate popovers

When GtkApplicationWindow has a menubar, we don't chain up
to the GtkWindow size_allocate, which used to position the
popovers. Move that to _gtk_window_set_allocation() which
is always called by GtkApplicationWindow.

https://bugzilla.gnome.org/show_bug.cgi?id=736205