Caused by rB608d9b5aa1f1: UI: Add shortcuts for constraint panels

Prior to rB608d9b5aa1f1, the constraint was gotten using context [CTX_data_pointer_get_type(C, "constraint", &RNA_Constraint) -- which is valid for bones on hidden layers].
After rB608d9b5aa1f1, the constraint is found (or isnt) using edit_constraint_property_get [this is not valid for bones on hidden layers because internally BKE_pose_channel_active checks if the bone is on an active layer].

Some observations:

• Every operator using edit_constraint_property_get doesnt work for bones on inactive layers [delete, moveup, movedown, move to index (drag n drop nowadays)]
  • moveup, movedown, move to index check if they could find a constraint beforehand though (dont crash)
  • delete crashes (doesnt check if a constraint could actually be found)
• Every operator using edit_constraint_property_get for constraint data doesnt work for bones on inactive layers [stretchto_reset, limitdistance_reset, childof_set_inverse, ...]
  • these all check if they could find a constraint beforehand though (dont crash)

This is because the poll function is using context to get the constraint, the operators themselves use edit_constraint_property_get which leads to inconsistent/unexpected results.

Possible solutions:

• [1] let the delete operator just work with the context constraint again (like prior to rB608d9b5aa1f1) -- allows for deleting constraints on bones in inactive layers
• [2] check if we could get a constraint -- prevents the crash, but does not allow for deleting constraints on bones in inactive layers
• [3] make the poll edit_constraint_poll_generic be as strict as the operators -- dont use context to get the constraint, but something like edit_constraint_property_get
• [4] make the operators be more graceful and let them act on bones on hidden layers -- let edit_constraint_property_get actually use context

Whatever we decide upon: this might be a candidate for 2.90.1? Will check on all solutions, might go for the quick fix [1] first, then try [4]?

Note: Adding constraints also doesnt work for bones on inactive layers [that was the case in 2.79 as well -- it is also using BKE_pose_channel_active]
Note: edit_constraint_invoke_properties also uses said context

Good find @Philipp Oeser (lichtwerk). I think it's important to have the poll function actually in sync with the rest of the operator; without that, it's rather useless. And first doing [1] and then trying [4] sounds good to me.