overview: Don't expect time argument from drag-end signal

We actually don't get a time from the xdndHandler when it emits drag-end, so we fail right now when calling workspaceManager.get_workspace_by_index(time).

Fix it by getting the time ourselves instead.

Fun fact: Looks like this bug was introduced with the commit that created the xdndHandler in 2011, back then drag-begin had a time argument and drag-end didn't. The signal handlers in the overview expected it the other way round: ceedc7e3