The aim of this work is to be able to provide custom subset string editor
GUI according to the layer. Typically, so that a WFS layer uses the same
editor than in its select source, or that a plugin can provide a custom
editor.
* Add QgsSubsetStringEditorInterface: abstract interface to define a dialog
that can edit a subset string
* Make QgsQueryBuilder implement QgsSubsetStringEditorInterface
* Add QgsSubsetStringEditorProvider: interface for thos who want to provide
a dialog to edit a subset string.
* Add QgsSubsetStringEditorProviderRegistry: keeps a list of subset string
editor providers. Transposed from QgsDataItemGuiProviderRegistry
* Add QgsGui::subsetStringEditorProviderRegistry()
- Make test() method virtual so it can be overriden in a derived class
- Make enabling/disabling of "use unfiltered layer" checkbox automatic
when layer's subsetString is changed (for example by an overriden
test() implementation)
- Add a codeEditorWidget() method that returns the sql editor widget,
so that custom behavior can be added.
