Improves search entry and exit behaviour (Fixes #52)

Given that the CategoryListWidget is already an extension of Gtk.ListBox, I figured that was the best place to add the capability for it to remember what it's last selected row was.

I've then hooked the triggering restoring the remembered selection into what seems to be the best place for when exiting the search is performed - but not when the search is just empty but still active.

Finally, I've set the title to "Search Result" when activating the search as this string is already in the .po files.