Add iconPath property to QMailAccount

IconPath will store a QString() containing a path to a icon that
can be used by email clients to easily distinguish between different
accounts.

Change-Id: I6addbddf59fd80301fd24b0b19e99b9bd5140133
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>