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

explorer_ECIQ8z0cPH.mp4

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:

• 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.

[WizardCM]

My only very minor concern is that MediaSliderMoved might run more than 60 times a second, which could cause the whole UI to lag while dragging. Wouldn't mind a side-by-side, but otherwise I'm OK with this change.

[RytoEX]

A future problem/concern/refactor might be to redo this so that we store the media's duration once when it's loaded and refer back to the stored value rather than pulling a strong reference (sometimes two!) and fetching the source's duration every time we update labels. However, if we decide that a "file that has its duration change while loaded" is a valid use case, then we would have to load its duration every time. That said, I'm not sure if the get duration function call actually gets a new duration if that's happening.