PlayerToolbar: Spread out widgets

Instead of grouping widgets into 3 boxes, spread them out, set the
minimum and maximum character widths to artist and title labels, and let
the progress scale expand.

This ensures that the labels will not get very small widths that they
could only show the ellipses by distributing empty spaces to the labels
and progress scale.

https://bugzilla.gnome.org/show_bug.cgi?id=699807