Change mesh importer to export mesh files with relative paths where applicable

[adamkewley]

Estimate: 1d

The mesh importer currently exports models with absolute mesh paths.

The reason it does this is so that the file is open-able through other tooling (official GUI, scripts, etc.), where that other tooling may have different heuristics for handling relative paths (e.g. the official GUI might look in Geometry/ whereas a script may not).

[adamkewley]

#172 is related, because it will probably (like this) be integrated as a gear/options wheel next to the "Export to OpenSim Model" button, where the user can select how they want to handle paths.

[adamkewley]

Must be part of a major release because it constitutes a visual change.