Sync Renderer: Fix the computation of the FPS used by the ISR.

We need to use the average FPS not the currently measured FPS.

Not doing that could lead to wrong timings for subtitles using frame-based timings.

Fixes #4857.
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.
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...