Keep authentication dialogs open until authentication completes

[Alberto Mardegan]

Currently, when performing a reauthentication request the dialog stays open only for the time needed to capture the user input; once the user has confirm the data, the dialog is immediately closed, and the data is being sent back to the authentication plugin.

However, it may happen that the authentication plugin is able to verify whether the data is correct or not (actually, in most cases it is --UbuntuOne, ownCLoud, OAuth...), and in case it's not correct it typically asks for another signon UI dialog to be opened, to request the new data.

To avoid re-popping up dialogs every time, it might be a better user experience if we keep the dialog open, with the fields disabled and a spinner running, until the authentication plugin has completed the operation.

This could be implemented with no changes to authentication plugins: instead, we would just modify signond to always call the cancel method on signon UI once it gets the result from the authentication plugin.