appDisplay: Only use dragMonitor for one icon at a time

appDisplay: Only use dragMonitor for one icon at a time

Instead of adding a dragMonitor for every icon in the grid as soon as
one icon is getting dragged, only add a dragMonitor for the icon that is
currently being dragged over (ie. the current drag-target). With a large
number of icons in the iconGrid, this should significantly reduce lags
while dragging.

We can do this by detecting the DnD-entering of an icon or folder using
the `handleDragOver()` callback of drag-targets, adding the dragMonitor
because we know an icon is hovering above the drag-target and then
detecting the DnD-leaving of the drag-target by using the `dragMotion()`
handler, where we remove the dragMonitor again as soon as the
targetActor is no longer our actor (ie. the drag-target).

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/849