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/platform/helpers.cc

@@ -30,41 +30,9 @@
 #include <mujoco/mjxmacro.h>
 #include <mujoco/mujoco.h>
 #include "engine/engine_vis_visualize.h"
-#include "xml/xml_api.h"
 
 namespace mujoco::platform {
 
-mjModel* LoadMujocoModel(const std::string& model_file, const mjVFS* vfs) {
-  mjModel* model = nullptr;
-
-  if (model_file.empty()) {
-    auto spec = mj_makeSpec();
-    model = mj_compile(spec, 0);
-    mj_deleteSpec(spec);
-  } else if (model_file.ends_with(".mjb")) {
-    model = mj_loadModel(model_file.c_str(), 0);
-    if (!model) {
-      return nullptr;
-    }
-  } else if (model_file.ends_with(".xml")) {
-    char error[1000] = "";
-    model = mj_loadXML(model_file.c_str(), vfs, error, sizeof(error));
-    if (!model) {
-      return nullptr;
-    }
-  } else {
-    char error[1000] = "";
-    auto spec =
-        mj_parseXMLString(model_file.c_str(), nullptr, error, sizeof(error));
-    if (!spec) {
-      return nullptr;
-    }
-    model = mj_compile(spec, 0);
-    mj_deleteSpec(spec);
-  }
-  return model;
-}
-
 void SaveText(const std::string& contents, const std::string& filename) {
   std::ofstream file(filename);
   file.write(contents.data(), contents.size());