The code assumed that the focus window was always the one at the
top of the window stack, which is not true if an unfocused window
has the above hint set.
Rather than fixing this assumption, rename the function to
lower_beneath_grab_window() and use the display's grab window - the
function is only used for displaying the tile previews, which means
that we want the grab window anyway.
https://bugzilla.gnome.org/show_bug.cgi?id=650661
If mutter is going to be a "real" library, then it should install its
includes so that users can do
#include <meta/display.h>
rather than
#include <display.h>
So rename the includedir accordingly, move src/include to src/meta,
and fix up all internal references.
There were a handful of header files in src/include that were not
installed; this appears to have been part of a plan to keep core/,
ui/, and compositor/ from looking at each others' private includes,
but that wasn't really working anyway. So move all non-installed
headers back into core/ or ui/.
https://bugzilla.gnome.org/show_bug.cgi?id=643959
