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/ui/tileseteditor.h

@@ -73,6 +73,8 @@ private slots:
 
     void on_comboBox_metatileBehaviors_activated(const QString &arg1);
 
+    void on_lineEdit_metatileLabel_editingFinished();
+
     void on_comboBox_layerType_activated(int arg1);
 
     void on_actionExport_Primary_Tiles_Image_triggered();