Enable changing sort_bef_combine parameter key

Attached patch re-implements the change from #2372781.

This patch fixes the #states value of the new form element.

This patch changes the state to use visible instead of enabled.

I set it as RTBC while also taking the change of setting it as a Bug since it falls into the category of "Regressions in functionality from previous major versions or stable releases (this may include missing functionality or APIs if there is no obvious replacement)".

Without this patch, there is no way of having two different views in the same page, with both using BEF and providing a Sort exposed filter since then we get the error described in 🐛 An illegal choice has been detected - GET requests with BEF combined Needs work .

Change will need test coverage and maybe schema updates.

Reroll for 7.0.x.

Patch for 7.0.x worked fine too.