Main menu entry "About Briar GTK"

Sub issue of #17 (closed).

changed milestone to %GTK 0.1.0-alpha1

added T: feature label

mentioned in issue #17 (closed)

changed the description

mentioned in merge request !7 (merged)

added S: in progress label

assigned to @nicoalt

mentioned in commit 56f846d3

mentioned in commit 56553840

closed via merge request !7 (merged)

added R: fixed label and removed S: in progress label