createOddShape label positioning fix

Previously labels for horizontal segments (bar the first) were set to
be centered on the line as opposed to keeping them on the 'outside' of
the shape. Now labels are correctly put either above or below.
New exercise for calculating side length of rectangle-composite shapes
Enhancements to hints, presentation and variety

Number of side range increased, added hints and changed scaling to
avoid awkard label positioning.
Merge pull request #39755 from davemajor/composite-shapes

New exercise: Rectangle Perimeter 2