[feature-request] add key bindings to adjust file tree size dynamically

Currently the width of the file tree is hard coded in the config. Most of the time this works very well. However, sometimes it's just too small and I'd wish to be able to dynamically increase the file tree width using some key binding.

I have now realized that this can be achieved using mouse dragging, so maybe my aproach in #148 is not the right one and one could just add key bindings to the existing mouse dragging code.