mac-virtualcam: rename signal into activate/deactivate

[sandboxcoder]

Description

Fix issue in SLOBS frontend app where it displays a modal dialog because it thinks the mac-virtualcam has "started".

Motivation and Context

Not a user facing bug. However, when I renamed the VIRTUAL_CAM_CONNECTED signal into "start" that triggers the issue on OBS frontend that makes it think virtual cam started which in turn causes a modal dialog to display when developer tries to close the app which introduces undesired delay in development.

How Has This Been Tested?

Verified mac-virtualcam works in both SLD and SLOBS frontend. The "Start VirtualCam" button is no longer automatically selected. No more modal dialog shown when you try to exit the SLOBS frontend

Types of changes

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the streamlabs 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.