map: add ULAplus palette support

[jorgegv]

Enable a different ULAplus palette for each map screen, which will be activated and deactivated when entering/exiting screen.

Also, add needed flow rules/checks so that a given palette can be activated/deactivated in response to certain events: item grabbed, random events (e.g. lightning), etc.

Tasks:

• Add DATAGEN support for definition of ULAplus palettes
• A global table of palettes which can be used anywhere with C functions and the ID of the palette, which indexes into the palette table
• Extend Map screen data struct with the ID od the associated palette for each screen
• Enhance map drawing code to take this palette info into account
• Add FLOW actions to change ULAplus palette
• Document everything