Rework double click handling in GtkTreeView.  We cannot blindly use
the 2BUTTON_PRESS and 3BUTTON_PRESS events.  In case a user does two
fast double clicks, we receive a 3BUTTON_PRESS and BUTTON_PRESS.  We
cannot easily deduce two double clicks from this.

We have removed the bookkeeping using row references of the last paths
clicked.  Instead we monitor event time, (x, y) coordinates and compare
against double-click-time and double-click-distance ourselves.