Draft: Switch System Tray item on hover

Now, hovering over a system tray applet will switch to that one without needing to click, provided the conditions are right:

1. The popup is already open
2. The popup is not pinned
3. The visible view is a single row/column layout

SNIs are skipped as they have no popup content.

Here's what it looks like in action:

simplescreenrecorder-2021-05-05_20.20.59

Implementing it was surprisingly simple. However this is a somewhat speculative MR to see if we really want to do it, since several things need to be fixed before we could consider merging it:

• Stop showing tooltips on hover when the popup is already open. The active property seems to do nothing at all!
• The Night Color applet needs to be either rewritten to be an SNI, or given content in its FullRepresentation.
• For some reason, the Battery & Brightness applet does not load battery details when activated via hover the way it does when clicked!
• Probably need to implement a triangle mouse filter so diagonal cursor movement into the applet doesn't unintentionally switch the view.
• Possibly visually group or separate SNIs from applets so it doesn't seem weird that some random-seeming tray icons don't do anything when hovered?

cc @teams/vdg @kmaterka @niccolove