keyboard-page: Use the apply mechanism instead of the next-page signal
assistant: Remove unused enum / "direction" parameter

I believe this became unused when using the new transition type in GtkStack.
assistant: Remove next-page signal
assistant: Unprefix gis_assistant_switch_to

It's not public API.
Don't needlessly ref/unref in apply
account: Split out "local accounts" section into a separate file

This will pave the way for the rewrite of the accounts page flow.
account: Do the same for the "enterprise login" section
account: Use a stack, not a notebook
account: Move the prepare_func to another file

We're going to split the account page into separate, interconnected pages.