deviceprovider: Add default implementation of _start()
This default implementation just does a static probe and emits a message for each device. So the new codepath where you can just call start without doing a probe before should also "just work" even for providers that just have a static list.
This fixes the regression introduced by gst-plugins-base!427 (merged) and closes gst-plugins-base#720 (closed). This should also obsolete gst-plugins-base!442 (closed).
Fixes #720 and gst-plugins-base#780 (closed)
Edited by Olivier Crête