Add a way to name metatiles, for use in c code

Metatiles can be given a name. The name becomes a constant in
includes/contstants/metatile_labels.h.

The plan is to be able to reference metatiles in code using a macro
like `METATILE(Building, TV_ON, Primary)`, which will evaluate to the
value 0x003, or `METATILE(BrendansMaysHouse, MOVING_BOX_OPEN, Secondary)`,
which will evaluate to the value 0x270.

include/core/metatile.h

@@ -3,6 +3,7 @@
 
 #include "tile.h"
 #include <QImage>
+#include <QString>
 
 class Metatile
 {
@@ -12,6 +13,7 @@ public:
     QList<Tile> *tiles = nullptr;
     uint8_t behavior;
     uint8_t layerType;
+    QString label;
 
     Metatile *copy();
     void copyInPlace(Metatile*);