OpenMW-CS: startup screen

When you open or create a content file in OpenMW-CS, all you are greeted with... is a blank window.

We could make use of this space by showing a list of available subviews to open. Just like the menu bar at the top, except that you could get to an element with just one click instead of two.

(RM-2824 from redmine: created on 2015-08-03 by scrawl ., )