Add keyboard navigation

[jcbvm]

The tree should be navigatable via arrow keys at any time (with for example the alt meta key). Focus should remain where it is, unless a translation field is focused and no leaf node is currently selected. In the latter case focus should return to the first translation field when a leaf node is selected again.

See #15

[jensmtg]

I was looking for this functionality, but it took me some time to figure out the hotkeys (alt-meta) to do this. Maybe add a line or two to the documentation?

[illeb]

It doesn't seems to work for me: if i understand the committed code correctly, i need to press left ALT and then press arrow keys to navigate in the keys trees?

I am using the .exe version of 2.0.0-beta-1