Add a menu entry for the periodic table
Add a menu entry for the periodic table

It can serve as a useful reference.

Change-Id: Ifaa4781c8f5ebc90e3dd8da5c8d1d64a89b019fe

wadejong/avogadroapp,wadejong/avogadroapp,wadejong/avogadroapp,OpenChemistry/avogadroapp,ghutchis/avogadroapp,OpenChemistry/avogadroapp,ghutchis/avogadroapp,ghutchis/avogadroapp,OpenChemistry/avogadroapp,ghutchis/avogadroapp,OpenChemistry/avogadroapp