Arrow key navigation support

This PR adds arrow key navigation support to the databrowser in the central panel.

Test: Select a dataset which has a lot of images, select one image by clicking on the thumbnail (so that the central databrowser panel has the focus) and then navigate in the dataset using the arrow keys on the keyboard. Also test the same with screens/plates.

(It's a bit of a hack, as it directly uses the UI components, but it works...)

--no-rebase
probably s/unless/if/
:+1: on explanatory comments, though!