Tileset Overrides: Make overridden tilesets configurable

[TheGrop]

Adds an optional field to tileset.xml ("override") that specifies the type of override for a tileset. There are several (case-sensitive) types:

• "Pa0:", where subtype is one of ("normal", "underground", "snow", "lava") - subtype is used for the colour of the blocks
• "Flowers" - for tilesets like Pa1_nohara
• "Forest Flowers" - for tilesets like Pa1_daishizen
• "Lines" - Simple lines
• "Full Lines" - Simple lines and more complicated lines (like circles)
• "Minigame Lines" - Complex lines for Pa3_MG_house_ami_rail