Refactor Plugin Handling

[Schlaefer]

• Moves plugin handling out of Phile\Core\Bootstrap into Phile\Plugin\
• Moves plugin class autoloading before configuration loading. Classes provided by plugins are accessible in configuration.
• Instead of global access to PLUGIN_DIR, event-bus and configuration pass those as parameters into Phile\Plugin\AbstractPlugin
• Allows multiple plugin-directories
• Moves test related files from plugins\phile\testPlugin to tests\fixture