[applets/systemtray] Make icon configuration UI clearer

Right now it's not clear that it's referring to the panel icons only, or that you can effectively make the icons auto-scale with panel thickness by choosing the largest size ("Enormous").

This MR adjusts the UI to make those facts more obvious. The "Enormous" size in the combobox is removed, and instead this setting is accessed by choosing the new "Scale with panel width/height" setting.

Before:

After:

cc @kmaterka @teams/vdg