+ Removed gfxdata.cpp file.
+ Removed get_gfx() from modules/gfx.h/cpp.
+ Added skin parts for each removed graphics of gfxdata.
+ Added IButtonIcon interface and an implementation for skin theme.
+ Removed "icon_buttons" from gui.cpp.
+ Now icons in button are set through set_gfxicon_to_button function.
+ Removed from Theme class check/radio_icon_size member variables
(they are replaced with the new IButtonIcon interface).
+ Removed jdraw_inverted_sprite(), now each icon has it normal/selected
version in the skin sheet.