compositor/plugin-manager: Split start() from new()
In order to support dynamic imports, gjs added an implicit mainloop that can drive the main context independently from other mainloops like the one from GApplication or MetaContext.
That means that sources can now get dispatched to the main context from the moment the plugin is started, resulting in a crash as the association between compositor and plugin manager doesn't exist until meta_plugin_manager_new() returns.
Make sure this doesn't happen by only starting the plugin after meta_plugin_manager_new() has returned.