0.5.8: Custom components, easier PoF translation, and other fixes

0.5.8 makes moving PhysicalOffsetFrames a little bit easier when they're the child of a Joint, and adds user-facing support for OSC-specific OpenSim::Components (explained below). It also contains a few minor bugfixes, plus a variety of internal engine changes.

• Editing the position of a PhysicalOffsetFrame that's a child of a Joint now behaves more intuitively (rotation still sucks, though)
• OSC-specific experimental OpenSim components are now exposed to the user via the model editor. So, as a user, you can now (e.g.) manually add a PointToPointEdge to your model in order to measure the distance between two points (previously: PointToPointEdge was only available through the Frame Definition UI)

Beware: OSC's custom/experimental components are incompatible with OpenSim. The intention
of this feature is to provide users with a way to test rapidly-developed OSC components
that may, with enough interest, be upstreamed to opensim-core.

• The available socket options when adding a new component to the model should now show all valid options, rather than just showing frames in the model (#820)
• The camera's location/fov is now editable in degrees
• The camera's panning behavior is better when viewing a 3D scene via a viewport with a very skewed aspect ratio (e.g. when the model visualizer is tall and narrow, or short and wide)
• Updated Sphinx to version 7.2.6, and the associated sphinx-book-theme to 1.1.0, which makes the documentation look a little nicer
• Internal: the engine now uses strongly-typed angles (osc::Radians and osc::Degrees) to reduce unit mismatches in the source code
• Internal: the test suite now automatically exercises all available UI tabs (mesh warper, model editor, etc.) tabs to ensure they are always working
• Internal: all header files are now suffixed with .h to match most libraries etc. (previously: .hpp)