Now the widgets factories can give a score on how good they could handle
a widget.
Additionaly, plugins can be added to choose a widget factory in function
of an external information. One of them uses a table in PostgresQL to
allow specification of the widget type and configuration.
I took the opportunity to remove a few deprecated method in relation to
this.
* It's now implicitly shared, meaning that changes to the config
can be supervised and a signal is sent from QgsVectorLayer and
there is no risk of elements suddenly being deleted.
* Remove a bunch of methods that were in QgsVectorLayer for legacy
reasons.
Do not rely on the presence of a dot to load
a module. Loading a module can now be achieved
through the code editor in the dialog.
* added an option to load from environment
* added a file selector to specify the file
* changed tooltips accordingly
