Create gui themes

[Bellog]

[SFGUI branch] Create default themes for every sfgui widget, e.g. windows widget.
Themes are saved in themes.txt file under src/gui directory. The file is automatically loaded in GUI.cpp
Example theme file: example

Notes:
Theme properties are CSS alike with the difference that they are written in CamelCase instead of lisp-case.
Expression Button#close applies to the button with "close" id.

Also try to find out what is possible to achieve using themes for future reference.