Add HdyPreferencesWindow
I need to check the code one last time and there are remaining "debuging prints" here and there. The docs likely need to be double checked too. So no need to rush reviewing it for the moment, I submitted the MR to document progress on this widget.
This depends on !253 (merged).
This could benefit from !256 (merged), but it's not mandatory and it can easily be added later.
Fixes #101 (closed)
Edited by Guido Gunther