experiment with splitted views based on karsten's input.

The goal is to support both the emacs usecase and the
usecase with fixed views for certain sets of files. The changes
gives the user more control where the files are. Effectively
it should be easy now to put your cpp files in the left pane,
and the header files in the right pane, and use F4 to toggle
between them.

Also make the 'x' in the toolbar close the document, and not the view.