use QgsFieldExpressionWidget in data defined properties

This uses the new field expression widget in data defined properties for a better UI coherence.
I had to const_cast QgsVectorLayer because QgsDataDefinedSymbolDialog uses a const layer pointer.
BTW, this fixes the fact that the help for the name symbol was not fully readable (was multiline).

@mhugent since you designed, can you tell me if it's ok for you?

new dialog:
![image](https://cloud.githubusercontent.com/assets/127259/2981110/6fe0d5e6-dbff-11e3-838f-8ad7a7b701f0.png)

old dialog:
![image](https://cloud.githubusercontent.com/assets/127259/2981097/3b527d20-dbff-11e3-98a3-02287f154b64.png)