[taskmanager] Fix previously-hovered windows getting highlighted on Wayland

On Wayland, if user clicks to activate a window in the tooltip, then opens the tooltip again, the ToolTipWindowMouseArea still thinks containsMouse is true, so the highlight is displayed again even though the mouse isn't actually hovering over the thumbnail, just over the icon.

Instead, we now use onPositionChanged which will only fire when the user really moves their mouse over the thumbnail.

I suspect using the positionChanged signal might be a bit less performant than containsMouseChanged? It would be better to make the MouseArea always have the true value of containsMouse, but I haven't found any way to do that. This solution is a bit hacky.