[applets/systemtray] Use a grid for the hidden items view
requested to merge ngraham/plasma-workspace:grid-view-systemtray-expanded-representation into master
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