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/stack.c

@@ -29,11 +29,11 @@
 #include <config.h>
 #include "stack.h"
 #include "window-private.h"
-#include "errors.h"
-#include "frame-private.h"
-#include "group.h"
-#include "prefs.h"
-#include "workspace.h"
+#include <meta/errors.h>
+#include "frame.h"
+#include <meta/group.h>
+#include <meta/prefs.h>
+#include <meta/workspace.h>
 
 #include <X11/Xatom.h>