Make WebUI and API code cleaner (+ 1.5 fixes)

[Picus303]

This PR aims to make the code of the WebUI and API cleaner by spliting them into multiple files instead of having one giant file for each.

There is now a structured folder tools/webui that contains the same code as before but in a more readable way.

I'd like this PR to be the first of a many that'll aim to make the project easier to maintain.

IMPORTANT: This PR is not complete yet and only contains the new code for the WebUI. The API is probably broken right now. If anyone have a few samples of code that I could run to test it, I would be glad :) The new code for the API should be ready quickly (I hope).

Personal note: I'd really like to help this project, but the giant files and use of global variables everywhere make it really difficult. Please, if it's possible, help me to help you by spliting your files :)