Simple API to change settings

[JelleBouma]

This PR implements a feature described in Issue #864

This adds:
A JSON Patch API that can be used to change any settings at runtime through an API.
Two new settings (not visible in GUI): bool UseApi (off by default, as this feature should be opt-in), and int ApiPort (the port to be used by the API, default 51111)

Example of an API call to change the theme to "Windows XP Silver":
PATCH http://localhost:51111

[
  {
    "op": "replace",
    "path": "/Theme",
    "value": "Windows XP Silver"
  }
]

[dremin]

Hi @JelleBouma, thanks for working on this!

I think a web server is a bit of a heavy way to implement this functionality and introduces new security concerns.

Have you considered using window messages instead? RetroBar could register a message using RegisterWindowMessage, and other applications can also use this to send messages to RetroBar. The only complication there is sending the string of the theme name, since we can't receive a string from another process, however there are a few options:

• Use WM_COPYDATA to copy the string value cross-process
• Instead of sending the theme name as a string, instead the other app could modify the RetroBar settings.json directly, and the window message simply acts as a notice to reload the settings.

[JelleBouma]

I will look into WM_COPYDATA.
I had considered faving the other app modify settings.json directly but it seems like a bad idea due to the unnecessary complexity of having to deal with a large file (whose definition likely changes with each release) to change only specific values.