applets/systemtray: fix tab focus after pin button
Reason for the change
The tab focus in the expanded representation uses a rather complex assignment where KeyNavigation.backtab in the loaded representation is assigned to the backtab of its container. This actually works in the backtab direction quite well, but breaks poor KeyNavigations implicit reverse assignment, which does not get sufficiently updated when a new representation is loaded. Depending on the precise circumstances, it often ends up still assigned to the previously loaded representation which obviously can't be focused anymore, so focus just ends up stuck in tab direction.
Instead, just set tab focus explicitly to the first entry in the representation's focus chain (or the hidden item view's, if that one is loaded). This is not always the optimal tab chain experience by itself, but it's at least something generally reasonable that individual applets can build on.
Test plan
- click on a systray item with a representation popping out (e.g. volume)
- click on another systray item with a representation and tab around
- (without) gets stuck on pin button (with) doesn't get stuck on pin button
Screenshots or screen recordings
No meaningful visual change