x264enc: allow lossless compression (qp=0)

https://bugzilla.gnome.org/show_bug.cgi?id=725051
xingmux: add muxer category and make element plugable

This is required to make encodebin consider the element. Yet it is not enough to
have it plugged it has a lower priority than most mp3encoders and no different
caps.
docs: install prebuilt plugin docs if gtk-doc is disabled

Sync to the Makefile.am from gst-plugin-base where it is done right.
Fixes #725034