libobs: frontend: Update monitoring to bool and add hotkey

[Warchamp7]

Description

Deprecates obs_monitoring_type in favor of an on/off boolean toggle. Adds source hotkeys for toggling monitoring on/off on a source.

Motivation and Context

#12735 and #13378 introduced easier access to monitoring in the UI, and made it so that monitoring is independent of mute state. This makes the MONITOR_ONLY state effectively obsolete, so this PR removes the enum in favor of an on/off state that represents the new behaviour.

How Has This Been Tested?

Tested that sources handled monitoring correctly, when set via the UI or via websockets using the now deprecated methods. Ensured that the new signal as well as the deprecated one were emitted and handled correctly.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

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