core: improved getOrClone + use in chat and widgets_handler: - getOrClone is now trying to re-use others instances of a widget if they have no parent, and will create a new widget only if nothing is found - it is now used in widgets_handler's changeWidget - it is now used in Chat factory, on existing widget - new host method deleteUnusedWidgetInstances remove instances without parent. It is used by getOrClone.

-e git+https://github.com/kivy/kivy.git#egg=kivy
-e hg+https://repos.goffi.org/sat#egg=sat
-e .