Add initial Emscripten support for Player

There are issues with function pointers yet. This initial approach allows to build and reach these issues.
Because JavaScript is event based and game loops block the browser, the loop function is passed as a callback to emscripten_set_main_loop, which will manage the loop by itself. That's why later the while block is ifdef'd too. The loop needs to be cancelled by a special function too, as the loop set does not have a condition and runs infinite by default.