This features a near-total rewrite of PluginLoaders and some code associated with them.
Highlights:
- PluginManager->registerInterface() does not return anything, and now accepts a PluginLoader instance instead of a string.
- PluginLoader itself is drastically simplified. getPluginFilters(), enablePlugin() and disablePlugin() are now removed. loadPlugin() responsibilities are now solely confined to doing whatever is necessary to make the plugin's classes visible by the server, and does not emit log messages or check for data directories.
- PluginBase->init() and PluginBase->isInitialized() have been removed.
- Plugin interface now declares a signature for the constructor which implementations must comply with.
- Plugin interface now declares setEnabled().
