Gui Editor: Bitmaps

[Admins]

We have added "Bitmap"s to the gui editor: they work both as container and inside other containers (windows, other bitmaps).

To make it easy to use it's also supporting adding bitmaps by drag'n'drop if the image already exists somewhere in the Graal folder. However the vertical position from drag'n'drop operations is messed up on Mac right now, it will be fixed in the next version.