More interface cleanups.
More interface cleanups.

Add an explicit close action
Port "please save" dialog from Avo2 including key shortcuts
Use standard QKeySequence to avoid translation / UI issues
   (not every OS uses the same key shortcuts)

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