0.5.13: `Force` and `Function` visualization, easier joint center placement

0.5.13 adds support for visualizing force vectors in the model editor, the ability to visualize OpenSim::Function properties (e.g. the ActiveForceLengthCurve of a MillardEquilibriumMuscle2018), and easier visual joint parent/child/center placement in the model editor - along with a bunch of other smaller changes/fixes!

• The model editor's/simulator's 3D visualizers now have an experimental Show > Forces' Linear/Rotational Component visualization option, which adds arrows indicating how each OpenSim::Force in the model applies its linear/angular force component to each body in the model. This can be useful for debugging model creation or ExternalForces
• The model editor UI now has experimental support for viewing OpenSim::Function curves. This is currently exposed as an eye icon in the property editor panel (#695)
• Selecting an OpenSim::Joint that has OpenSim::PhysicalOffsetFrames for both its parent and child frames now shows a 3D manipulation gizmo that lets you move the joint center without moving anything else in the model (#159)
• ... and much more! (see CHANELOG.md in the main repository for more information)