Update etag of parent dir when adding/removing ext storage mount points

See https://github.com/owncloud/core/pull/11305#issuecomment-57150156

Fix for https://github.com/owncloud/core/issues/11008

- [x] Add hooks for add/remove mount point
- [x] Update etag for all applicable users
- [x] Unit tests
@icewind1991 do you think we should keep these in core ?
Back then I struggled to find the right arguments that make sense for core.

How reusable this is for example for the shared mount points ?
It seems the etag propagator is currently specific to external storage, I don't think we want to use the "markDirty" mechanism for sharing, so perhaps we should move the signals to files_external to make them specific to that ?
What do you think ?