Modernize the AccountEditor.

This now uses libadwaita for everything, but will need some updates when
libadwaita 1.2 is released. Avatars are currently non-functional as the existing
buddy icon API leaves a lot to be desired and doesn't work for what we're trying
to do here.

Testing Done:
* Created new accounts with the new dialog.
* Modified accounts with the new dialog.
* Modified accounts with the old dialog and verified the changes showed up in the old dialog.
* Created accounts with the old dialog and verified they were displayed properly by the new dialog.

Bugs closed: PIDGIN-17592

Reviewed at https://reviews.imfreedom.org/r/1737/