Load plugins after Workspace is created

The original intention behind creating plugins before the workspace was
to handle the case where kwin_wayland may need to wait until outputs are
available. However, since things have changed a lot in that regard,
plugins can be loaded after the workspace now.

The main benefit behind this is that plugins can be simpler, they won't
need to track when the workspace is created.

On X11, plugins are already loaded after the workspace is instantiated.