systemtray AbstractItem: Improve touch feedback

Applets in PlasmoidItem were blocking events from reaching the ToolTipArea, so I moved the on hover logic to the AbstractItem's MouseArea and put the MouseArea over the iconContainer. The iconContainer is the parent of applets. Presses tend to hit the center, which is where the applet is, preventing ToolTipArea::containsMouse from being true and preventing the highlight from moving to the pressed item.

Hopefully this doesn't cause duplicate actions when events reach the applet. I tested the patch on the nightcolor applet and all the other default applets and didn't see anything wrong. I think this may be because the signals are sending QML MouseEvents to the applets, causing them to be accepted if the applet can accept them. Then when the signal handlers are done, since the events are already accepted, they should not move beyond the MouseArea and cause another round of event handling.