This commit hides the GdkDisplayManager instance and class structs,
adds vfuncs for listing displays, opening displays, and getting and
setting the default display. The X11 backend has a derived
GdkDisplayManagerX11.

The gdk_display_manager_get() function is responsible for deciding on
which of the compiled in backends to use. Currently, it consults the
GDK_BACKEND environment variable and falls back to x11.

Dropping those will be an unnecessary pain.

All backends updated.

Use the grab and ungrab vfuncs from the frontend instead of the
_gdk_windowing wrappers, and move some things around accordingly.
Again, only the X11 backend has been updated, other backends
need to be updated to match.

And nuke GdkDeviceManagerPrivate at the same time. Again a commit
that only deals with the X11 backend, other backends will need to
catch up.

It turned out no vfuncs were necessary. I've decided to move
the screen member up to GdkVisual, since it is the same in all
backends. The X11 backend subclasses now, to add the X members
that it needs to keep track of. GdkVisual and GdkVisualClass
are hidden now.

Otherwise we run into warnings from xdnd_status_filter

Nonstatic backend-specific functions need an x11 in their name.

This commit hides GdkDragContext and GdkDragContextClass, adds
vfuncs for most drag context functionality, and turns the X11 DND
implementation into GdkDragContextX11. We also add vfuncs to
GdkDisplay for gdk_drag_get_protocol and to GdkWindow for
gdk_drag_begin, and implemenet them for X11.
Other backends need similar treatment and are broken now.

Add a GdkDisplay::get_app_launch_context vfunc, and a
gdk_display_get_app_launch_context that for X11 returns a subclass.
For win32 and quartz, the implementations were trivial, so we
just return a new GdkAppLaunchContext without subclassing. Since
the type of the context now depends on the display,
gdk_app_launch_context_set_display is deprecated.

After this commit, pc files no longer include the gdk backend in
their name, and we no longer install a pc file for gdk. There is
now a single gtk+-3.0.pc file. It turns out a separate gtk+-x11-3.0.pc
file is not necessary, since gdkx.h doesn't have a separate include
directory that would have to be added to Cflags.

This commit does a number of things:
- remove some dead wchar configury from configure.ac and gdkconfig.h
- repurpose gdkconfig.h as header that contains GDK_WINDOWING_foo
  macros for each included backend, include it in gdk.h and install
  it in $includedir instead of below $libdir
- drop the backend from the library names
- build libgdk-3.0.la as a convenience lib and include it in libgtk-3.0.la
It does not yet enable building multiple backends at the same time.

This lets us change this without affecting apps.

There is no need for apps to access this class, as its only implemented
in the gdk backends, and we want to be free to change it later.

Now a cell can either have a "fixed" size or it can have
an "aligned" starting point or both. "fixed" size cells take
no space when they are invisible.

Added _gtk_cell_area_set_cell_data_func_with_proxy() to be called by
gtk_cell_layout_set_cell_data_func() when the layouting object itself
is not the underlying cell area.

Even if the default CSS contains values for these, handle non-existing
values as queries might happen on an incomplete style.

https://bugzilla.gnome.org/show_bug.cgi?id=637606

https://bugzilla.gnome.org/show_bug.cgi?id=637606