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
Decapitalize two layer names necessary for bigmap localization
Provide text boxes for LoW bigmap localization

This also fixes an opacuity issue with the heightmap