Common: refactored GUILabel
Common: refactoring GUIButton: applying coding style and tidying up
Common: refactoring GUIButton: split drawing procedure

This splits GUIButton's drawing routine into four parts: Draw, DrawImageButton, DrawTextButton and DrawText.
The placeholder type initialization has been moved from drawing to SetText method to avoid parsing same string every time the button is redrawn.
Common: refactored GUIObject