Separation of the transitions dock from the main window

[tytan652]

Description

Moves the transitions dock UI outside of the main window UI.

Also reorganise/cleanup transition setters and events in OBSBasic (which includes frontend API related things).

Motivation and Context

The awaited sequel of #7278, and it's the real one (so also the sequel of #11880).

How Has This Been Tested?

Please tests

Types of changes

• Tweak (non-breaking change to improve existing 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.