Scrollbar: IWD2 has a transparent scrollbar

its not a big deal to redraw the window for all scrollbars since before we were calling core->RedrawAll() anyway
Scrollbar: take image resources at construct time
GUIScript: take scrollbar resref for creation
GUIScript: remove GemRB_ScrollBar_SetSprites
ScrollBar: remove SetImage
GUIScript: move game checks to their own module
GUIScripts: add ability to wrap CreateXXX functions
GUIScript: extend and wrap CreateScrollBar to allow passing sprite indices

We want to limit the GameType checks to GUIScripts, but we dont want to have to manually pass hardcoded indicies through the CreateScrollBar function. Use the wrapper ability to pass the correct indicies to GemRB_Window_CreateScrollBar
Merge branch 'scrollbar'