Unused beam geometry and MLC parameters remain editable for inverse planning

[LiBu981]

Current behavior:
When a plan is set to "inverse", some beam parameters should be made unavailable in the UI.
For example for the pyRadPlan dose engine only the gantry and couch angle are currently used, making it confusing for the user that other parameters remain editable.

Expected behavior:
These unused parameters should be disabled on choosing inverse planning (or should this be adjustable for different dose engines?).