Keyboard navigation (up/down) when inline editing TreeTable contents

[CarstenWickner]

As an AIS user I want to be able to easily switch between edited cells (up/down) when configuring the names of assignable categories in order to not having to click-out of the currently edited one and click-into the next one. Instead pressing the up/down arrow keys should go to the next field above/below the edited one and start editing that immediately.

A nice-to-have would be to allow for the tab/shift+tab keys to do the same as down/up arrow keys respectively.

This maybe also relevant for the HermeneutiX related configuration if inline editing is introduced for some parts, e.g. #14 and/or #15.