Added GetPort method (closes #46)

[bminer]

I noted in the documentation that GetPort() only works after the Window is running (i.e. Show() has been called)... but, to be honest, I'm not sure that users will fully grasp this. One option is to return an error if webui_get_port returns 0. Thoughts?

[AlbertShown]

Every time we call .show() webui use a different random port for security reason, and this port get used only when a window is started, and a web server is running. But we can set our custom port if we want.

[AlbertShown]

One option is to return an error if webui_get_port returns 0. Thoughts?

Yes, we can do that 👍