Users may end up having both PySVN and Subvertpy installed on the same

system. A previous change had SVNTool favor PySVN over Subvertpy, but

it's useful to be able to specify which one specifically should be used

(especially for development and testing).
The list of possible backends is now specified in

settings.SVNTOOL_BACKENDS. It can be overridden to choose a different

backend (even a custom one, in theory).
This will also form the basis of new multi-backend unit tests.

Tested swapping the backends, with some logging to see which client was

used. The unit tests ran with the custom backend.
Tested emptying the list, which resulted in SVNTool being unusable due to

missing dependencies (same as if neither backend's dependencies were installed).
Tested setting the list to a non-existent module.
Tested setting the list to an actual module that was not a SVNTool backend