Add a dconf setting for enabling/disabling notifications BZ#738465

Edited the gsettings schema to include a checkbox option for
enabling/disabling notifications, and added an 'if' in
application.py to check the value before enabling notifications.

The value name is 'notifications'.

This commit fixes BZ#738465.

https://bugzilla.gnome.org/show_bug.cgi?id=738465