Disallow deleting a blank tile

[Asuna Yuuki Nagato]

Steps to recreate:


Open 'Select Tile' window'
Click a blank tile
Press delete key
Observe "Confirm Delete" Yes/No msgbox.

This can create the confusing impression that the tile isn't actually blank. Consider having trying to delete a blank tile display the "Remove Tiles" msgbox instead.

[Asuna Yuuki Nagato]

That msgbox is created by delete_tiles in zq_tiles.cpp