expression button and expression capability added to the field model

This follows #1289

It allows adding expressions to the field model.

An expression button has been created (buttons can be promoted in Qt designer). It uses the expression dialog and adds directly the new expression to the model.

One thing, I don't get: in QgsFieldModel::data, it seems that for combo box (only thing I tested), the font role is never asked. I wanted the expressions to be in italic, but that doesn't work.