Allow changing window layout

[woutgg]

Usually I keep the application on one side of the screen and I don't like making the window too big. The problem with that currently, is that the column widths cannot be changed and not disabled either. This has the effect that the sources/favourites column becomes too narrow (thus cutting of text), while the 'Rooms' column is taking up space even though I have no use for it at all.

So, it would be handy to be able to resize, and perhaps also disable columns.

[pascalopitz]

Happy for pull requests on this one.