In cases where two controls sit alongside one another such as a label
alongside an edit box, I've previously been arranging for them to be
vertically centred by fiddling with the size request and alignment of
what I assume will be the shorter control. But now I've written
columns_force_same_height(), that's a much easier approach, and it's
also compatible with GTK3 without using a deprecated method; so this
change switches over all vertical centring to doing it that way.
Also, while I'm here, I noticed that the edit box and button of
CTRL_FILESELECT / CTRL_FONTSELECT were not vertically centred, and
since it's now really easy to make sure they are, I've added another
use of columns_force_same_height() there.
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