display of crossing bonds

[rwst]

From shk3
https://sourceforge.net/apps/trac/jchempaint/ticket/36

In cases where one bond is drawn crossing over another one (but not linked to it), it would be helpful if the bond at the bottom were shown broken (Marvin does this). Also, once the crossing bond has been drawn it is then impossible to modify it (e.g. to turn it into a wedge or double bond).