UI: Update media source time labels while seeking

[Warchamp7]

Description

Update the time and duration labels in the context bar when changing the seek position

https://github.com/obsproject/obs-studio/assets/1554753/a044da07-351e-43e5-9cf2-681e42236f7e

Motivation and Context

Better feedback of user interaction

How Has This Been Tested?

Seeked back and forth through multiple media files.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

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