GUI: Don't clip surfaces with a drawable area
GRAPHICS: Add support for setting a vertical drawableArea
GUI: Add mode to skip drawing of button for PicButton
GUI: Horizontal scrollbars

This commit adds support for horizontal scrollbars. It also adds support
for triangles rendered pointing left and right.
GUI: Introduce ScrollableCanvasWidget
GUI: Add images for unknown cover-art
GRAPHICS: Add function to return if drawable area is empty
GUI: Grid