Use display_name in Libmessagingmenu indicator
Instead of using the 'nickname' property of
AccountInformation, which was removed with commit 1bccd51a "Clean up
ServiceInformation APIs" use
With MR !145 (merged) the Libmessagingmenu indicator is actually being built again, but apparently it missed the
AccountInformation.nickname removal. This should fix that and make MR !145 (merged) compile.