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()
