Improve handling of selected activity in the overview.

[GeraldJansen]

After switching to the Gtk.TreeView for the overview, there are a few glitches concerning selection of previous activities:

1. the selection doesn't survive the automatic update of the overview (every 60 seconds)
2. movement with the Up and Down keys is quirky: it would be nice to restore Hamster behaviour (Up starts from bottom, and Down starts from top)
3. double clicking on an activity should open the edit window, like Ctrl-E