expression button and expression capability added to the field model
QgsFieldCombobBox do not allow expression
    editExpression protected, setLayer uses QgsVectorLayer
    make epxression widget editable
remove protected slots in sip
fix signal emitted twice
handle null expression (fix empty row)
coherent color (colored text, not background)
fix wrong slot
QgsFieldModel::layer() returns QgsVectorLayer
field model only works with vector layer, add convenience slots for QgsMapLayer in QgsFieldExpressionWidget and QgsFieldComboBox
typo in var name
add signal with expression validity
graduated symbol: do not auto classify on field changed (prevent error dialog at each keypress)
update sip
Merge pull request #1318 from 3nids/fieldexpressionmodel

field expression widget (with expression capability added to QgsFieldModel)