Version 0.96 release.
New creature window made without use of H3 graphics

This is import of ancient unfinished branch from svn. As of now window is largely finished but commander-related elements are still TODO.

Note that I also uploading graphical content - this is intended and necessary for final goal - to make VCMI functional without any additional graphical pack.
Button overlays are now more generic (not necessarily text), removed old version of creature window
Leveling up commander skills seems to be working. Optimized some png's to decrease size
Fixed levelling up of commander
Refacoring, tweaks.
Correct zone placement & sizing for underground maps.
Correct placement of subterranean gates.
Implemented leveling up abilities. TODO: implement visible selection/descriptions
Updated readme to with info on assets license, including link to not-yet-existing vcmi-assets repository
Fixed texts:
- added new texts to translate.json
- implemented texts for abilities

Remaining TODO's:
- handling of stack artifact
- handling of commander artifacts
- stack experience
Somewhat better gate placement, attempt to add rocky tiles to underground zones.
More tweaks.
Correct underground terrains. More improvements for connection placement.
Implemented stack experience/commander experience display for window
Better gravity-based algorithm.
Throw exception when there are not enough two-way monoliths available.
Fixed gravity algorithm not attracting adjacent zones at all -.-
Extracted too big lambda into a separate helper method
Implemented stack artifact handling, no tested yet