applets/kicker: allow keyboard navigation to favorites sidebar
The system action and application favorites on the side of the kicker popup are currently not accessible by keyboard, and if they were, they are not screen reader accessible.
This change implements tab- and arrow key navigation from and to the favorites sidebar (with a single tab stop for the whole list), and adds screen reader accessibility for the entries. To implement this, the icon is changed to a ToolButton (from a base item) and the position-dependent sidebar layout is changed from a rather complex state-based anchor mechanism with plenty of hardcoded values to a straightforward layout. The change to a ToolButton introduces a visual feedback change; rather than just having a brightening effect, hovering the icon now has the typical bounding box highlight for ToolButtons.
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Access from the search field directly to the favorites sidebar is currently missing as I touched the same code in !3015 (merged) and don't want to do it twice - if one gets merged, I'll update the other MR.