UI: Add flag/file to disable built-in updater

[derrod]

When distributing OBS via third party platforms that have their own update systems we want to be able to disable the OBS updater without having to resort to having a separate build entirely.

This is a PR for one of the changes proposed in RFC #⁠30: obsproject/rfcs#30

Description

This change adds a new --disable-updater command line options and /disable_updater(.txt) sentinel file to disable the built-in OBS updater.

Motivation and Context

As per obsproject/rfcs#30, when distributing OBS via third party platforms the built-in updater should be disabled to avoid conflicts with the platform's updater system.

To keep the maintenance effort to a minimum we'd like to re-use the existing build and simply include the sentinel file or start parameter in the platform configuration to disable the updater.

How Has This Been Tested?

Compiled on Windows 10 and verified that the "Check for Updates" option is greyed out and the update check on startup does not happen.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[dodgepong]

Didn't this exist a while ago, and was removed after the v21.1.0 debacle?

[derrod]

No, that was a CMAKE variable, i.e. a build time option. This is entirely done at runtime.

[dodgepong]

Ah, my mistake.