Add SignOnUiContainer to signon module

This type allows clients to specify a container item into which
the signon-ui service can embed UI for user interactions, instead
of spawning a new window.

This commit also improves the documentation for the signon process.
Fix various bugs in account models

Previously, changes to or removals of an account would only be
detected by the model if the account was created during the lifetime
of the model.  This commit ensures that all such changes in the
database are detected by the model.  It also fixes removal-related
bugs in the service accounts model.
Merge pull request #58 from chriadam/master

[nemo-qml-plugins] Improve signon adapters with signon-ui container