leaks: add weakref capability to Instruction, and introduce .proxy_ref for Widget and Instruction. This allow us to use the instance without having a reference to it. Keep track of all the callbacks created in kv for each widget, and unbind them when the widget is gone.
animation: use widget.uid instead of widget, to prevent to store weakproxy.
widget: fix equality between widget and proxy by comparing proxy only
widget: add __self__ to get the real reference, even if we have a proxy.
tabbedpanel: keep the reference of our all the content-childrens somewhere.
Merge pull request #1318 from kivy/fix-kv-leaks

Fix kv language leaks (and some others)