Enhance the backup/restore capabilities of Repository Explorer

ModeShape offers full repository backup&restore functionality via the RepositoryManager API. Using this in practice requires clients to write code atm.

We should add backup & restore buttons to the repository explorer, which would make the whole backup/restore process much more easy. The backup button should initially present the user with a file-chooser for the output folder of the backup, while the restore button should allow the user to select the folder from where to load the backup data.
Once MODE-2440 is implemented, the backup action should also allow the user to check/uncheck whether binaries should be included.