By putting the Control into a ScrolledWindow, we get scroll bars whenever the Control's container is smaller than its minimum size.

Vertical scrolling of the ScrolledWindow currently is deactivated, as we still have our custom implementation for vertical scrolling.

This, however, raises some issues: At first, the vertical scroll bar gets scrolled away along with the Screenplay Control while scrolling horizontally, as it is part of the contents of the ScrolledWindow.

Also, the hand-made scrollbar lacks some features, such as auto-hiding or the "precise scrolling mode" on Gtk.

Eventually, the custom scrolling implementation is probably going to get in our way when implementing  limburgher#20 anyway, so we should try to port vertical scrolling to the ScrolledWindow scrolling mechanism.

Before this commit, horizontal scrolling also moves the vertical scroll bar.

I specifically implemented fd5c593 to make this comment not necessary