[HTML5] Locale, input fix, context, exit.

Add missing semicolumns in engine.js
Add optional extra args to JS Engine.startGame
Remove loader.js, explicit noExitRuntime.
Also add onExit callback (undocumented in emscripten)

platform/javascript/detect.py

@@ -164,3 +164,6 @@ def configure(env):
 
     # callMain for manual start, FS for preloading.
     env.Append(LINKFLAGS=["-s", 'EXTRA_EXPORTED_RUNTIME_METHODS=["callMain", "FS"]'])
+
+    # Add code that allow exiting runtime.
+    env.Append(LINKFLAGS=['-s', 'EXIT_RUNTIME=1'])