windowPreview: Unset selected if window was clicked to switch workspaces

This is used to detect whether a click was short enough to select a window and activate it or long enough to start a drag. Usually when clicking on a window and selecting it, this leaves the overview, but when clicking on a window on a neighboring workspace, the overview is kept open, but selected is not unset in this case. So all attempts at dragging the window after using it to switch workspaces will fail.

Fixes #3783 (closed)