Cleanup model loading flow.

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

src/experimental/studio/app.h

@@ -130,8 +130,9 @@ class App {
     std::string last_print_data_file;
   };
 
-  void OnModelLoaded(std::string_view model_file);
+  void ClearModel();
   void ProcessPendingLoad();
+  bool IsModelLoaded() const;
 
   void ResetPhysics();
   void UpdatePhysics();
@@ -173,6 +174,7 @@ class App {
   platform::SimProfiler profiler_;
   platform::SimHistory history_;
 
+  mjSpec* spec_ = nullptr;
   mjModel* model_ = nullptr;
   mjData* data_ = nullptr;