pulse: let pulseaudio define prebuffer/minrequest and use that as buffersize, also change maxlength to 100ms

It's more along pulseaudio recommendations, and we don't really need to define those
ourself. Maxlength is defined to keep latency in someway in limits. Please tell if
this acts for you.

This will do for now to meet popular demand

"Advanced Open File" menu opens a "media" chooser, an "Open Media" named
dialog. Keeping "Open File" is wrong then.
Compared to this menu, Open "File", "Network", "Capture" and "Disc" are
just 'shortcuts': They needed to be grouped then.

This temporarily removes CAM support (which was reportedly broken a few
days ago). Scan is left for the time being.

Use cqam:// as BDA already does.