These two functions center selected frames. When you click on the
Center horizontally
icon, all selected frames are moved up or
downwards until all of them fit an imaginary common horizontal line
with their physical horizontal centre lines. This common horizontal
line is the center line of an imaginary rectangle that covers all
selected frames. The horizontal (X) position of the frames is not
altered by this alignment.
With Center vertically
the frames are moved to the left or
right until they fit an imaginary common vertical line; their vertical
(Y) position will not be altered.
