add theme functionality for background day/night earth map image

Each theme is a directory under ./data/themes, and E:S is shipped
with themes 'default' and 'vector', the former being the hardcoded,
required default.

User theme selection is stored in preferences file, and may be set
either at Options Screen or via command line --theme option.

A theme directory structure is conceptually similar to ./data:
it contains subdirectories /images, /fonts, /sounds, etc.
Currently theme only affects the background earth day/night image
of the map screen, so only /images exist.

However the full set of game images, including arrow and icon, must
be present in each theme. This is now done by symlinking default theme
images to vector. Files must have the same name, but type and extension
may be different. Currently only pngs and jpegs are accepted.
fix Sri Lanka in vector background
DO NOT MERGE THIS BRANCH YET! See TODO.txt