Add OpenSim::AbstractSocket::canConnectTo(Object const&) const

[adamkewley]

Currently, the only way to test whether a socket can be connected to an object is to call AbstractSocket::connect(Object const&) in a try {} catch {} block and see if any sparks fly. This is problematic if (e.g.) you need to write a UI that lists all objects in an OpenSim::Model that can be connected to, because that requires iterating over many objects and (slowly) throwing many exceptions.

That use-case was triggered when developing OSC's socket editor (issue). I got around it by caching a list of connectees once, but it would be better to avoid exceptions altogether.

[adamkewley]

Fixed in the PR