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

simulate/macos_save.mm

@@ -0,0 +1,33 @@
+// Copyright 2022 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <sstream>
+#include <string>
+
+#include <Cocoa/Cocoa.h>
+
+std::string getSavePath(const char* filename) {
+  NSSavePanel* panel = [NSSavePanel savePanel];
+  NSURL* userDocumentsDir = [NSFileManager.defaultManager URLsForDirectory:NSDocumentDirectory
+                                                          inDomains:NSUserDomainMask].firstObject;
+  [panel setDirectoryURL:userDocumentsDir];
+  [panel setNameFieldStringValue:[NSString stringWithUTF8String:filename]];
+  if ([panel runModal] == NSModalResponseOK) {
+    std::ostringstream s;
+    s << [panel.URL.path cStringUsingEncoding:NSUTF8StringEncoding];
+    return s.str();
+  } else {
+    return "";
+  }
+}