Move the installed includes to a subdir

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

src/core/boxes.c

@@ -27,7 +27,7 @@
  */
 
 #include "boxes-private.h"
-#include "util.h"
+#include <meta/util.h>
 #include <X11/Xutil.h>  /* Just for the definition of the various gravities */
 
 /* It would make sense to use GSlice here, but until we clean up the