Refactoring of GUI classes and replacing of essential bits of graphical set

Large changeset, can be split into 3 interconnected parts:
1) Creature window. Pretty much what I had in #22 . Notes:
* All graphical files were replaced with those made from scratch
* Bits like bonuses icons are now in a separate submod of vcmi (will upload on forums after merge)
* One missing piece - commander artifacts. Will finish after merge since proper approach here requires large changes in artifact UI classes.

2) New graphical assets.
* All essential elements from "vcmi mod" have been replaced with my own/public domain images
* All resolutions but 800x600 are now in a separate submod. Everything else is now in a submod.
* Graphical "sources" will be uploaded into a separate repository named "vcmi assets" or something like that.

3) Refactoring of GUI classes with large changes in code.
* All classes that represent windows are now in client/windows directory.
* All classes that represent UI elements/widgets are now in client/widgets
* API cleanup in button-related classes. Button is finally named as such, created small helper classes for common needs (like class that executes function on click) instead of creating custom classes every time.

Looks to be ready for merge although I won't consider it completely finished - will get back to all of this after some bugfixing & work on Launcher (this pull needs better submods support)