model/view for layer/field selectors

[3nids]

This implements a model/view for selecting layers and eventually their corresponding fields.

The models can be used directly on top of widgets.
Anyway, I implement sub-classes of QComboBox since the connection between layer and fields models can only be made by the selection model. And, having the field model changed by a change in selection in the layer combo is not a good idea (it must be changed on currentItemChanged and not selection changed).

For testing, I implemented these model in two places:

• DXF export: a checkable list of layers
• Atlas: a layer combo related to a field combo

Also, my intention is to extend the field model and combo to handle expressions (since it is used in many places).

Thanks a lot to @NathanW2 for his initial python model: https://gist.github.com/NathanW2/d2bf29d9043c5658dab2
and to @wonder-sk for his advices.

[wonder-sk]

Do we need two signals for this? I would prefer to drop the signal with "int" parameter. From experience in QGIS, using field indices instead of names always led to some troubles. Best to use only field names and update the code that uses field indices to use names.

[wonder-sk]

The code looks good now, from my testing:

• atlas: coverage layer combo box also shows check boxes - it shouldn't
• atlas: sorting by attribute does not seem to work

[3nids]

It's weird for the checkboxes in the combobox, I don't have them (ubuntu).
Moreover, I checked in the debugger, the flags are correctly set (without ItemIsUserCheckable).
Do you have any idea where does it come from?

I just fixed the problem for sorting by attributes.