[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.

wpigui/src/main/native/include/wpigui_internal.h

@@ -24,6 +24,7 @@ struct SavedSettings {
   int yPos = -1;
   int userScale = 2;
   int style = 0;
+  int fps = 120;
 };
 
 constexpr int kFontScaledLevels = 9;