New property: emptyComponent

Currently, we can define an 'emptyText' component that will be centred in NMRium if no spectrum is displayed. However, this still assumes that we can 'drag / drop' some data and therefore the layout shows the dashed line with the text in the middle.

In the case of simulation or prediction, I would prefer to have a fully customizable component that will also replace the dashed lines. This would allow me to create a user guide for the simulation and prediction pages.


@targos is this ok to add this new property `emptyComponent` ?