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
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
Implemented stack experience/commander experience display for window
Extracted too big lambda into a separate helper method
Implemented stack artifact handling, no tested yet
Cleanup of stack artifacts code
Moved some code from GUIClasses into two smaller files:
- CArtifactHolder.*
- CComponent.*

Goal: reorganize artifacts UI to decrease dependency on CGHeroInstance class
Reducing size of GUIClasses. Moved Garrison Int into a separate file.
Reducing size of GUI classes. Split trade windows into a separate file
Moved all non-window GUI elements out from GUI classes file. TODO: reorganize source tree
Added missing files
Reorganized client source tree:
- client/widgets for reusable GUI elements
- client/windows for independent windows
- client/gui for base GUI classes which should remain internal
Cleaning up includes
Merge branch 'feature/creatureWindow' into refactoring/guiClasses
Fixed compilation issues caused by merge