Add optional yalign argument to Utils::mk_label() method

As part of the work on bug 652044 - uses deprecated APIs, selectable
vertical alignment was defaulted to ALIGN_CENTER for all labels.  The
relevant commits can be viewed in comment 26 of said bug report.
https://bugzilla.gnome.org/show_bug.cgi?id=652044#c26

For multi-line labels a vertical ALIGN_CENTER value is not
consistently aesthetically pleasing.  This becomes obvious when a
single-line heading label is paired with a multi-line value label.
To improve the aesthetics, a vertical alignment of ALIGN_TOP is
preferred.

Hence re-add the ability to optionally specify a vertical alignment for
labels.  If a yalign value is not specified a default vertical alignment
of ALIGN_CENTER is used.