effects/overview: Show windows in the hovered virtual desktop
It's inconvenient if a user only wants to know what windows exist in a virtual desktop, but is disappointed after hovering on the virtual desktop just to find that nothing happens.
This commit adds a timer to fix the problem. The timer is started when the mouse hovers on a virtual desktop item, and is stopped when the mouse leaves the zone or a window is dragged into the zone.
When the timer is triggered, a new property named
assigned to the hovered virtual desktop, and the model of WindowHeap is
updated to show windows in the hovered virtual desktop.