UI: Move media control dynamic properties to ui file

[cg2121]

Description

This moves the dynamic properties for the media control buttons from the cpp file to the ui file.

Motivation and Context

Nicer code

How Has This Been Tested?

Opened OBS and made sure media buttons still looked correct

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

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.