qt: proper tab and backtab navigation

Currently, Qt determines the tab and backtab key navigation targets. This does not work as intended in most cases. In this merge request, I tried to define the navigation targets properly (at least the main ones), so that besides arrow keys users may also use tab and backtab keys.

The main advantage of this is navigating within a view such as grid view. Currently, if you want to navigate out of a view, you need to go through all items until hitting the edge. This is no longer necessary with tab/backtab.