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/display-private.h

@@ -34,11 +34,11 @@
 #include <glib.h>
 #include <X11/Xlib.h>
 #include "eventqueue.h"
-#include "common.h"
-#include "boxes.h"
-#include "display.h"
+#include <meta/common.h>
+#include <meta/boxes.h>
+#include <meta/display.h>
 #include "keybindings-private.h"
-#include "prefs.h"
+#include <meta/prefs.h>
 
 #ifdef HAVE_STARTUP_NOTIFICATION
 #include <libsn/sn.h>
@@ -99,7 +99,7 @@ struct _MetaDisplay
    * class is constructed.
    */
 #define item(x)  Atom atom_##x;
-#include "atomnames.h"
+#include <meta/atomnames.h>
 #undef item
 
   /* This is the actual window from focus events,