Implement notifications

Meta issue:

• #28 (closed)
• #29 (closed)
• #57 (closed)

added T: feature label

mentioned in issue #28 (closed)

mentioned in issue #29 (closed)

mentioned in issue #30

changed milestone to %GTK Next

When testing Briar (0.1.0-alpha1), one user expected notifications to show up for new messages and therefore replied later than wanted.

I didn't replied to you because I didn't note that you sent me a message.

mentioned in issue #57 (closed)

changed the description

removed milestone

closed

changed the description

changed milestone to %GTK 0.1.0-beta2