Fixed glitches with thick black label text
Made the script more resistant to breakage

Does no longer require the labels layer to contain both black and yellow
pixels, and produces a clear error message if it's a layer group.
Added English language text labels for The South Guard to speed up the map localising process

Also incorporated the labels_flat.png into the .xcf file.
Bold text for all labels, minimum size 18pt

Text was illegible in 800x600 resolution
Integrated English text labels into AOI map
Added English text labels to remaining campaign maps
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.