GUI: Add Finnish translations, reword constrain buttons, and fix minor issues

This MR will add long-awaited* Finnish translations to ASE's GUI.

During translation, a few issues were encountered and fixed:

• Add localization to "Delete selected atoms?" popup.
• Fix localization of "Enter a chemical symbol or the name of a molecule from the G2 testset: [long list]" helper-popup, and do not print the "G2 list" part when not relevant (for example, building a nanoparticle from molecules seems not supported). Actually, it seems that the allow_molecule option of the widget is used nowhere in GUI. Remove unused allow_molecule option of the Element widget and update the helper message.
• Remove non-existent graphene.py from po makefile (file removed in 2cc2bce8).
• Reword "Constrain selected atoms" buttons to "Fix selected atoms" to clarify that the button applies the FixAtoms constraint. (This could be left as it was, but should then the translation specify the actual behavior "fix" or follow the ambiguous "constrain"?)
  • Other related buttons are also reworded a bit so that constraints.py and settings.py use the same wording. Both of these appear to encode the same functionality, so one of them could be removed?
• Remove "Constrain immobile atoms" buttons that do nothing, resolving one wtf from the code.
  • The desired behavior might be achieved in GUI by using "Select immobile atoms" and "Fix selected atoms" buttons (not tested).

As a downside, these fixes will change the user interface a bit and result in some fuzzy messages in the existing translations.

* See IRC meeting notes from 2012-01-24.