* Improved plugin system to allow handling of different plugin system versions without problems:
- Each plugin installs a desktop file stating the plugin system version and the lib to load just like KDE does it.
- So far additional information is not used yet but it is documented and should be added.
* Removed the 3Store backend. It was incomplete and 3store does not support removal of single statements anyway.
* Added method Plugin::isAvailable to allow a plugin to do additional checks at run-time.
* The Sesame2 backend now checks if JNI loads properly in isAvailable()