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/workspace-private.h

@@ -33,7 +33,7 @@
 #ifndef META_WORKSPACE_PRIVATE_H
 #define META_WORKSPACE_PRIVATE_H
 
-#include "workspace.h"
+#include <meta/workspace.h>
 #include "window-private.h"
 
 struct _MetaWorkspace