This adds api in the provider gui to allow a provider to create a custom
widget which exposes options to customise and change the layer's source.
E.g. a file based layer provider could expose a widget for selecting
a new file path, and an online one could expose widgets for changing
the host, authentication, etc.
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()
