Add right-clicking the model's `ForceSet`, `JointSet`, etc. shows a corresponding `Add Force`, `Add Joint`, etc. menu

[adamkewley]

Estimate: 0.25-0.5d

Mentioned by @JuliaVanBeesel during a UX session.

At the moment, there's a top-level Add menu that lets the user add anything, but Julia's expectation was that right-clicking on a particular set (e.g. a constraintset) within the model should show a menu for adding an element (e.g. a constraint) to that set.

[adamkewley]

Dropping from 0.4.1 - a few new bugfix tickets (e.g. #645, #644) pushed time out of the schedule and I need to drop ~3 days of development time from the milestone.