DiscoverDrawer: impove sidebar keyboard navigation
The global drawer in Discover currently has inconsistent and partially undiscoverable keyboard navigation. Arrow keys work for the parts of the sidebar that represent categories, and they are reachable through the tab key. But the actions that are fixed (including Home, Settings, etc.) cannot be reached this way, only through accelerator shortcuts. While this makes them still keyboard-accessible, they are invisible to users relying on screen readers as they are outside the tab focus chain. And even for sighted users, the inconsistency impairs usability. Moreover, while it's possible to reach the search field using the arrow keys (by moving past the end), it's not possible to escape the search field with the arrow keys. Contrast with e.g. System Settings, where pressing down- arrow from the search bar moves focus to the other sidebar entries below in a seamless manner.
This change makes the manual ActionListItems part of the focus chain, which automatically makes the Kirigami arrow navigation work. Arrow- movement between the search field and actions below is hardcoded. A bit of apparently dead code that blocked arrow navigation past the Updates section (presumably the end of the list in a previous iteration of Discover) was removed as well, to allow full arrow navigation within the sidebar.
It would have been nice to have down-arrow stop at the bottom of the list; it instead wraps around to the search field, as that is next in the focus chain and Kirigami uses that to handle arrow keys in GlobalDrawer. There does not seem to be a clean way to overwrite that from outside. In any case, that's already existing behavior before this change.
Keyboard navigation is still a bit weird, especially with categories that open subcategories, but it should be a clear improvement for now.