Communicators

[jovanbulck]

To be merged after #106

Improved Communicator hierarchy to reduce hard coding and improve code re-use. Summary of changes:

• QuietCommunicator now provides the default info and error strings. Subclasses should focus on visualisation, and may override default info/error strings when needed.
• Re-implementation of the DialogCommunicator.
• Re-implementation of the minimalist SummaryCommunicator.
• Simple LoggerCommunicator that prints netlogin progress via the logger module.
• Basic Qt GUI front-end that runs a kotnetcli Worker in a background thread and employs a custom communicator to translate netlogin progress calls to Qt signals, connected to the main GUI thread.
• Communicator-level credentials prompting and proper GuestCredentials.

[jovanbulck]

@GijsTimmers This can also be merged, after you've been able to review the changes of course.

[GijsTimmers]

Wow. Thanks for writing the code. Also, the Qt frontend definitely looks interesting.