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-private.h

@@ -25,8 +25,8 @@
 #define META_BOXES_PRIVATE_H
 
 #include <glib-object.h>
-#include "common.h"
-#include "boxes.h"
+#include <meta/common.h>
+#include <meta/boxes.h>
 
 #define BOX_LEFT(box)    ((box).x)                /* Leftmost pixel of rect */
 #define BOX_RIGHT(box)   ((box).x + (box).width)  /* One pixel past right   */