Refactor simulate to split UI from application

Fix name of glew library

Move simulate to new directory and refactor to use C++ conventions

Wrap simulate in mujoco namespace

fix namespaces, convert struct to class, use std::lock_guard

use GetMutex to better handle destruction ordering

rename mjSimulate to Simulate

nit: space before public and typo

move startup print

use nullptr instead of NULL

use GetInstance() to get simulate object

convert init to default constructor

sample/Makefile.macos

@@ -17,7 +17,3 @@ all:
 	clang++ $(ALLFLAGS)    derivative.cc -framework mujoco        -o derivative
 	clang++ $(ALLFLAGS)    basic.cc      -framework mujoco -lglfw -o basic
 	clang++ $(ALLFLAGS)    record.cc     -framework mujoco -lglfw -o record
-	clang   -c $(CFLAGS)   uitools.c
-	clang++ -c $(CXXFLAGS) macos_save.mm
-	clang++ $(ALLFLAGS)    simulate.cc uitools.o macos_save.o -framework mujoco -framework Cocoa -lglfw -o simulate
-	rm *.o