Refactor setattrdeep
Bugfix.

Why rewrap an already wrapped list? Furthermore, ordinary lists get
wrapped anyway in CallFrontListBox.
Removed redundant code.

Value set is overriden with the same value in connectControl.
Move getdeepattr to gui.
Refactor notify_controllers.
Move ControlledAttributesDict to gui module.
Move notify controllers to gui.
Refactor notify controllers.
Fix nofity_controler for nested components.
OWPlot extends OWComponent.
Moved getattrdeep to utils.
Warn when SettingsProviders are used without declaration.
Create a new instance of SettingHandler for each widget class.

When settings with name extended each other, both widget would use the
same SettingHandler, even if child widget defined additional settings.
Refactor widget constants.
Do not remove setting providers.

Extended classes will need them to initialize their setting providers.
Refactor SettingsHandler.