Let plugins handle dispatching

A plugin might need to add non-wl_display based dispatching, such as timeouts or idle callbacks. For this to be possible, we need to have the application integrate the "main loop" of libdecoration into its own. The common way of doing this is exporting a file descriptor for all events related to the context.

It must be the plugin that reads and dispaches events for the wl_display, as some potential future plugins (e.g. gtk, Qt) need to handle their own Wayland event processing.