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.)

[originally from svn r8177]