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/compositor/plugins/default.c

@@ -21,8 +21,8 @@
  * 02111-1307, USA.
  */
 
-#include "meta-plugin.h"
-#include "window.h"
+#include <meta/meta-plugin.h>
+#include <meta/window.h>
 
 #include <libintl.h>
 #define _(x) dgettext (GETTEXT_PACKAGE, x)