WebUI: use `filepatharg` in `onload` path.

Fixes #3296.
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new `[hasaudio]` info.

Conflicts:
	src/mpc-hc/res/web/controls.html
WebUI: show a "X" when there's no audio available.
WebUI: Cleanup.
WIP

Conflicts:
	src/mpc-hc/res/web/controls.html

Conflicts:
	src/mpc-hc/res/web/controls.html