Split account page into two

This commit begins to split password setting off from account
information. The password page is only shown for local accounts -
this information is communicated between the accounts page and
the password page by storing the local account name in property
of GisDriver.

The basics of creating accounts are working.

Still to do:
- Add an avatar chooser
- Get account data from online account
Prepopulate account page from online accounts

Take the name and the avatar picture from an online account,
and suggest them on the account page. For now, we only know
how to get this data from Google, and we need a patch for
gnome-online-accounts in bug 720446. This code should probably
be replaced by more advanced code that can get profile data
from other providers as well, together with the new avatar chooser.