Improve naming of theme properties throughout GUI code
Rename ItemList's bg -> panel
Rename ItemList's bg_focus -> focus
Rename ProgressBar's bg -> background
Rename ProgressBar's fg -> fill
Rename Tree's bg -> panel
Rename Tree's bg_focus -> focus
Rename ScrollContainer's bg -> panel
Rename FileDialog's *_icon_modulate -> *_icon_color
Rename FileDialog's files_disabled -> file_disabled_color
Rename CheckButton's on/off -> checked/unchecked
Rename check_v_adjust -> check_v_offset