Add cut, copy, and paste functions to Avogadro.

Also add missing type check in cjsonformat.cpp.

Still need icons, Steve will have them soon.

Change-Id: I35efb9a3ba55d746606222d0457e1b3b2eb80649
Merge topic 'copy-paste' into master

675c3b57 Add cut, copy, and paste functions to Avogadro.