GChemPaint canvas progress

The current stable GChemPaint version would display:

This is not consistent with usual chemist's habits, but was easy to implement using pango.

There are two issues: the minus sign should be the unicode minus sign instead of the more usual ascii character and, more importantly, it should stay above the stoichiometry coefficient. This is not possible using a single PangoLayout. I needed to split the text between several layout objects and now get:

Just if you are not a chemist, this string represents the phenylsulfonate anion.