[wpigui] Limit frame rate to 120 fps by default (#5030)

Limiting with vsync is apparently unreliable on a number of systems;
this resulted in high CPU/GPU usage.

Also add current actual frame rate to about dialog of GUI tools.

outlineviewer/src/main/native/cpp/main.cpp

@@ -183,6 +183,8 @@ static void DisplayGui() {
     ImGui::Text("v%s", GetWPILibVersion());
     ImGui::Separator();
     ImGui::Text("Save location: %s", glass::GetStorageDir().c_str());
+    ImGui::Text("%.3f ms/frame (%.1f FPS)", 1000.0f / ImGui::GetIO().Framerate,
+                ImGui::GetIO().Framerate);
     if (ImGui::Button("Close")) {
       ImGui::CloseCurrentPopup();
     }