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.