New property: emptyComponent

[lpatiny]

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 ?

[targos]

You just want to remove the dashed lines (and drop behavior) or also replace the grey rectangle? I'm asking because the current implementation is quite intricate and not easy to customize.