Implement conflict handling in the plugin loader
The plugin meta-data was introduced to allow plugins do define conflicts with other plugins, e.g. GStreamer 0.10 vs GStreamer 1.0
This was never fully implemented though. We can do this finally for the new tracker3 support
This needs adding a "Conflicts" setting in the .plugin file which upon loading is checked against the list of already loaded plugins. A message should be shown when loading the conflicting plugin