systray: Fix systray collapsing on click
The MouseArea in AbstractItem needs to be above applets when it's in the grid hidden area so that it can receive hover events while the mouse is over an applet
But when the applet is shown as an icon on the regular
systray it needs to be below so the PlasmoidItem onClick
takes precedence and systray collapsing works.
BUG: 449533 FIXED-IN: 5.24