Inlines the platform::LoadMujocoModel into the App class which simplifies
the logic and allows us to correctly display loading errors.
Also updates shortcuts:
Load Model (Ctrl+L) -> Open File (Ctrl+O)
Reload Model (Ctrl+R) -> Reload Model (Ctrl+L)
PiperOrigin-RevId: 840232553
Change-Id: I957d29046ae681d2aeb69310c3a2eec64d042d49