Show Inbox children on XLIST non-English accounts: Closes bug #729372

Although a prior fix dealt with the XLIST issue of a translated Inbox
name ("Posteingang") not being available when directly addressing the
folder ("INBOX"), it didn't deal with addressing children (subfolders)
of Inbox ("Posteingang/eBay") -> "INBOX/eBay").  This ensures Geary
can always identify Inbox, whether its translated name or INBOX.
Merge branch 'master' into wip/only-incomplete