0.5.19: Hotfix for STL mesh loader memory usage

0.5.19 is a hotfix release that's coming so soon after 0.5.18 because it patches a bug in the STL parser that causes models using STL meshes to consume a very very large amount of memory.

Additionally, it adds a MacOS-specific bootup icon, adds support for negative scale factors (a handy trick for mirroring meshes), makes it possible to toggle a single ContactGeoemtry component through its is_visible toggle, replaces the custom ExpressionBasedBushingForce
frame visualization with OSC's in-built frame visualization (which is toggle-able), and filters out any NaNed geometry coming from OpenSim (which can sometimes break OSC's hittest). See the CHANGELIST.md file for a full overview of the changes.