coregrilo: Properly load tracker plugin

All grilo plugins are loaded at startup. However, TrackerWrapper may
not be available yet. In that case, GrlTrackerWrapper is not
created. Once TrackerWrapper becomes available, all grilo plugins are
loaded again in order to instantiate GrlTrackerWrapper.
This approach has two issues:
- all plugins are activated two times
- TrackerWrapper may become available while the first tracker source
is added. In that case, GrlTrackerWrapper is instantiated with a first
source but a second source will be created by the second
load_all_plugins call (a new source is created everytime the tracker
plugin is activated). Then, GrlTrackerWrapper won't get any
notification because they will be associated with the second source
and not the first one.

Both issues can be solved by only (re)loading the tracker plugin when
TrackerWrapper becomes available.
2 jobs for wip/jfelder/coregrilo-tracker-plugin in 7 minutes and 45 seconds (queued for 2 seconds)