pavucontrol.glade previously set the channel label width to 15
characters, with the goal of making all channel labels have the same
width. However, with some translations and font settings the configured
width wasn't enough, so sometimes a label was wider than others, and
that made the volume slider widths different too. If the volume sliders
have different widths, it's very hard to visually compare the volumes of
the channels.

This patch removes the fixed width in pavucontrol.glade and solves the
problem by finding the widest label and using that label's width with
all labels.

Fixes: #51