Only reload contact list on changes

Currently, it's reloaded every time a message arrived, which produces a flickering list because for a tiny amount of time the selected contact gets unselected.

added T: bug label

changed the description

changed milestone to %GTK Next

mentioned in issue #80

added Usability label

marked this issue as related to #80

This should get better with #80, because then we can theoretically reload the list without getting the flickering, because the widgets are reused.

added S: blocked label

removed milestone %GTK Next