add options to count metatiles to tileset editor

- count the total number of usages across all maps of specific metatiles
- display the count and/or display unused metatiles
- this does not account for metatiles used in code (most of these are labeled)

include/core/maplayout.h

@@ -35,6 +35,11 @@ public:
         Blockdata blocks;
         QSize dimensions;
     } lastCommitMapBlocks; // to track map changes
+
+    int getWidth();
+    int getHeight();
+    int getBorderWidth();
+    int getBorderHeight();
 };
 
 #endif // MAPLAYOUT_H