Delete all messages of a chat

Related to #10.

added T: feature label

mentioned in issue #10

changed milestone to %GTK 0.1.0-beta2

mentioned in issue briar#1782 (closed)

mentioned in commit briar@fbe367f9

mentioned in merge request briar!1287 (merged)

mentioned in commit briar@2afbc7c9

mentioned in commit briar@7a62ce2d

mentioned in commit briar@11f56d50

mentioned in commit briar@6c6dbfd3

mentioned in issue python-briar-wrapper#20 (closed)

assigned to @nicoalt

mentioned in commit 69c5702a

mentioned in merge request !85 (merged)

mentioned in commit 9ed3bf41

mentioned in commit 04bb71bd

closed via merge request !85 (merged)