New Version 0.6.0 Nitrogen

Finally, and I apologize for the delays, a new version with quite a few fixes and improvements.
Even the snapshot version, which already had quite a bit of that in it still required some additional fixes, but now it should all be wrapped up. Enjoy!
New features:
- Levels on which items are drawn are accessible; bonds are indicated to cover other bonds if on higher level
- Broken bond indicator with same color as rest of bond
- Show sum formula of molecule during hover and after selection
- Closable wiki query
- Improved performance for drag-and-drop insertion of molecules
- Associated *.msm files with Molsketch
- Fixed crashes after undo/redo (changed linkage between molecule, bonds, atoms)
- Fixed global preferences (accidentally had no effect)
- Fixed number of hydrogens and charge in saving/loading molecules