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
The point there was that the KeyListener is attached to the component if it gets clicked on, in particular every time it gets clicked on, which leads to weird effects in terms of keyboard navigation, so I just made sure, that the KeyListener is only attached once to the component. Yes, not really a nice piece of code... but required the least changes.