Ability to remove the association of a repository with a project

Currently, the Synchronize With command shows the repositories with which a project has been synchronized in the past in a combobox. The combobox lists those repositories above the pseudo-line (-----) and all other repositories below the line.

Once a repository has entered the first group, it is no longer possible (barring manual edition of the prefs file) to return it to the second group.

I plan on adding a button for that action. The exact wording of its label ("remove this repository", "forget about this repository", "break this association", etc.) remains to be determined.