Hopefully final fix for MBTiles databases that fail to open.

Add counters for various reasons for fetching tiles to the Tile status popup (double-click empty space on the map).

Use PRAGMA application_id = 0x4d504258 to stamp newly created MBTiles files as MBTiles tilesets - Ref; https://www.sqlite.org/src/artifact?ci=trunk&filename=magic.txt