More image file & floor map handling progress

Added support for tiled images to TilePalettePair, added flag to force
palette entry 0 to be transparent, some loading and auto-detection
changes; various changes to GridEditControl, only part of which are used
in Yggdrasil at the moment; improved FloorMapEditor's map drawing code,
now loads actual on-screen map images from the game data to render icons
for stairs, treasure chests, etc., also improved wall drawing somewhat;
some other minor changes