We currently install two signal handlers for button-press-event and
selection-changed. As the former also calls the latter, the two interact
in non-obvious ways that can lead to two alerts being played at the same
time under certain circumstances.
Avoid this problem by setting activate-on-single-click on the tree view
and listening to the row-activated signal to drive previews and model
changes.
https://bugzilla.gnome.org/show_bug.cgi?id=671446
If the sound theme is set to be a custom sound theme, but it
does not exist, then we should fallback gracefully on the
default sound theme, and avoid warnings.
https://bugzilla.gnome.org/show_bug.cgi?id=645541
