Use display_name in Libmessagingmenu indicator

Christian Kellnerrequested to merge
gicmo/geary:libmmi_displayname into master

Instead of using the 'nickname' property of AccountInformation, which was removed with commit 1bccd51a "Clean up AccountInformation and ServiceInformation APIs" use AccountInformation.display_name.

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.

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