export properties to clipboard

[erjvd]

It would be nice to be able to export the property tables (bond, angle, torsion, ...). This was discussed a few years ago (https://sourceforge.net/p/avogadro/mailman/message/27569399/), but doesn't seem to be implemented yet.

[ghutchis]

I'm not going to implement anything in 1.2, but I have the code to do this in Avo 2.0 (i.e. table selections can be copied, etc.)