Run pyimgui in web browser

[NaleRaphael]

(Just a memo for this feature request)

In order to make this application be easier for people to use and accessible from web, we've made a plan to make the whole application runnable on web.

For an application implemented in pure C++, it's possible to run imgui in web browser, e.g. hef/imgui-in-browser. But a transpiler is required to convert C++ code to wasm, e.g. Emscripten.

Since our implementation is mainly based on Python, there are at least these difficulties:

1. As it's stated in this SO post, most of the Python-to-JavaScript transpilers are still at the early development stage.
2. We are using C extension built from pyimgui, and we are not sure that C extensions are transpilable.

In branch build_for_web, it's possible to run this application on repl.it. However, it's required to switch the backend from pyglet to pygame, and the performance on repl.it is not well enough to be published.