Add vector and field chooser widgets.

I have created these two helpers to generalize a wide spread feature: using widgets to display the layer list, and the field list associated to a layer.

The widget classes are general and the specific widgets are implemented in suclasses. I already implemented combos for both layer and field chooser.

So, lists, trees or whatever, with possibly multiple row selection can easily be implemented.

This will be very useful for plugins devs I suppose. I did not write yet the SIP files before I am sure everything is in order.

Please, comment or +/- 1.