Patch from James H providing lots more paranoid casting. Also one

Patch from James H providing lots more paranoid casting. Also one
actual behaviour change: Untangle now permits dragging with the
right mouse button, which has exactly the same effect as it does
with the left. (Harmless on desktop platforms, but helpful when
"right-click" is achieved by press-and-hold; now the drag takes
place even if you hesitate first.)

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@8177 cda61777-01e9-0310-a592-d414129be87e