i18n: Add comments for translators


Some strings are in Avogadro are hard to translate without context. For
example, what is “QM Matches”. Quantum Molecular Matches? No idea.

If you think that the meaning of a string is not 100% clear please add a
comment for translators. That works like this:

rbc = new QRadioButton(tr("Enabled", "Color frame"), this);

rbh = new QRadioButton(tr(“Enabled”, “Hue frame”), this);

In these case the translator would get context-information ("Color frame"
and “Hue frame”) in Linguist while the GUI would show “Enabled” in both cases
(when LC == English).