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Playback option page: Minor UI adjustment.

Ref: #4766.
Formats option page: Reworked UI.

Replace the text buttons by icon buttons and add a new button to clear all associations.

Also replace the "with icons" checkbox text with something clearer now that there is more space.

Fixes #4687.
Subtitles options: Clarify the "Delay interval" setting.

 - Rename it "Delay step".
 - Add a tooltip to explain what it does exactly.

Fixes #4865.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
Why "C:\tmp.srt"?
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...