Draft: login: use InAppNotification component to display errors on login

Add GtkOverlay and InAppNotification and remove error_message label in login.ui. The LoginError parsing and display impl is moved to fractal Error widget which replaces the error field in session. Function connect_prepared is updated to accept a function for the success case and one for when the error is present, which is taken and shown with the InAppNotification

Continuing the work for better error feedback, I added a simple InAppNotification to replace the previous error label and improved the error passing from the session using the prepared signal. Here's an example of the current state: