Split _gabble_roster_item_get into lookup and ensure versions
Split _gabble_roster_item_get into lookup and ensure versions

Use the ensure version everywhere, except in roster_edited_cb where we
don't really need to create an item just to see whether there are
unsent edits.

Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,jku/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble