Publicly, get objects for a variant rather than from it since we're
not actually getting the objects from the variant, just looking them
up using the variant as an id.
Internally, use `to_` rather than `get_` when converting between plugin
and engine objects, since its typically just a cast or a lookup to do
so.
Implement these in Application.EmailStoreFactory's implementation of
the interface.
This switches the variant format used to `(account_id,email_id)` to
match that used for folders, and so a folder doesn't need to be
specified when showing an email.
This breaks opening email from desktop notifications, but that will be
fixed shortly.
