Volume control of notification sounds

(cherry picked from commit aeb0ec980da771d4d321312b1f85a97e8dc6e23e)
Merge branch 'gnome-3.12' into gnome-3.10
Lower required libgnome-desktop version to 3.8
Do not warn about restoring time on first run

(cherry picked from commit 12090856b411fe8aceeb912ad4eb7f5f07750fa2)
Remove unused dependencies from install instructions

(cherry picked from commit b91d7263a48926b2253bbcc60e5004888809f748)
Merge branch 'gnome-3.10' into gnome-3.12