Add basic plugin-loading support

[adamkewley]

Estimate: 2-8d

Related to the fact that people may use OpenSim Creator to debug plugin creation:

https://pubmed.ncbi.nlm.nih.gov/32170995/

This might require quite a bit of work, because OpenSim Creator will effectively need to package a particular version of an OpenSim SDK alongside the installer so that people can build against it. OpenSim's ABI isn't particularly stable, so it will effectively be required that users compile their plugin against the exact version of OpenSim that OpenSim Creator uses.

[adamkewley]

This should be de-prioritized/dropped because there is no intention for OSC to ship OpenSim's C++ API - any kind of plugin API for OSC should be python-based and designed from scratch (rather than inheriting OpenSim's)

[adamkewley]

#884 required adding plugin code to the OpenSimCreator build, so now there's a OpenSimThirdPartyPlugins directory in the project where we can place in-demand plugin code from collaborators.

However, I'm going to close this because, again, there's no intention of adding a C++ plugin API to OpenSimCreator: it routinely updates opensim-core and changes compiler flags etc., so it would be a maintenance nightmare to maintain a stable C++ API for users.

Instead, the recommendation moving forward shall be to write a plugin for OpenSim (+ GUI), establish its usefulness, and then it can be included in-tree in OSC if enough people want it.