Bigmap localization: Decapitalize layer names

'labels' and 'label placement guide' have to be lowercase
Provide text boxes for LoW bigmap localization

This also fixes an opacity issue with the heightmap
Move old bigmap overlays to 'resources'

These overlays no longer appear in the game, but might stil be useful for
translators trying to localize the new bigmaps.

Overlays that have already been replaced aren't included.
Merge pull request #2 from aquileia/LoW_map

Bigmap localization