Deactivate & disable layer checkbox when no data is available for the given RFMO

[eblondel]

deactivate & disable layer checkbox when no data is available for the given RFMO (at now we handle a style, with a "fake" SLD rule) as 1st approach, or; evaluate the possibility to hide completely the layer in the layers panel (to discuss further)

This at now concerns:

• WECAFC: the 3 main layers (VME closures, other areas bottom fishing areas)
• SPRFMO: VME closures & bottom fishing areas

Behavior should be applied on switching to global view or reset, + switch to another RFMO view

[tdipisa]

We have to investigate more about this in order to avoid if possible too much additional request to check if data is missing.

[fabio2111]

Not a priority now.