Load FXML immediately when on FX Application Thread

When loading an FXML-file with a custom component as a child, the initialization of this child is currently delegated to a new Runnable which is posted to the FX-thread. This creates problems when the parent tries to access components of the child component, as these haven't initialized yet.

This race issue is prevented by performing the loading of the custom child component during the loading of the parent component, as this last action is already taking place on the FX-thread.