Requests: Add CloseMainWindow

[BenJuan26]

depends on https://github.com/obsproject/obs-studio/pull/8889

Description

Adds a new request called CloseMainWindow which closes the main window and cleanly shuts down OBS.

Motivation and Context

This request allows external applications to cleanly shut down OBS.

It implements https://github.com/obsproject/obs-websocket/issues/511

How Has This Been Tested?

Tested OS(s): Windows 10 Testing method: Postman

After connecting and handshaking, I sent this payload:

{
    "op": 6,
    "d": {
        "requestType": "CloseMainWindow",
        "requestId": "19b2e634-67b8-470a-a216-fc0f6a87160e"
    }
}

OBS (with the changes in the obs-studio PR linked at the top) responded by cleaning up its resources and shutting down. The events of the shutdown can be seen through the websocket:

Types of changes

• New request/event (non-breaking)

Checklist:

• [x] I have read the Contributing Guidelines.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] My code is not on master or a release/* branch.
• [x] The code has been tested.
• [ ] I have included updates to all appropriate documentation.