Multiple preferences management

The goal of the blueprint is to provide specifications to manage Inkscape's preferences so that it's possible to:
* save a version of the current preferences.
* reload a previously saved version.
* reset to the default settings.
* optionally manage corrupted preferences.xml files.

New name, description and last saved date fields would be required to list all available files in the default preferences location, but it can also be done (more easily) with a simple open/save dialog.