Replace Confirmations

[danirabbit]

This is a regression from the Gala notification server. Confirmations should also have a nice replace animation

[Marukesu]

i believe gala is the single sender of confirmation notifications now. given that, we can replace "x-canonical-private-synchronous" with "transient" and expire_timeout = 2000. gala keeps track of the used id so there's no real reason to keep supporting the canonical hint.