[applets/systemtray] Use a grid for the hidden items view

This improves the applet in a variety of ways:
- Much more touch friendly than the current skinny list view
- Many more items can be displayed without needing to scroll
- Substantial code simplification
- UI improvement from no longer sometimes showing an awkward vertical
  strip of icons that was invisibly scrollable but some of whose items
  ate scroll events, and which pushed the header over to the left in a
  somewhat random-looking way

BUG: 402681
FIXED-IN: 5.20