Allow 3rd party position sources

There appears to be a significant amount of interest in the provision of location information from 3rd party plug-ins.
This would be useful, for instance, for operators who would like to write a Cell Id location plug-in for their network and to pre-install the plug-in on network locked phones.Using plug-ins would probably be a bit disruptive at this point of time, so other options might need to be explored.

It also needs to be considered how users would be able to sensibly choose from the various plug-ins provided, or even if they get a choice at all.