Further fix for ``lang._handlers`` memory leak related to ``Bubble``.

Original problem: Bubble constructs BubbleContent before finishing its
own construction, thus never initializing its ``_proxy_ref``, causing
``lang.get_proxy`` to return the Bubble itself (this is when creating
handler for BubbleContent's color from parent color). This in turn
caused ``lang._handlers`` to unnecessarily keep it alive. 

The fix: I let ``Widget._proxy_ref`` be generated lazily on demand
instead of relying on ``Widget.__init__``: whenever the user calls
``Widget.proxy_ref`` for the first time, ``Widget._proxy_ref`` will be
initialized. I don't see any particular reason to initialize
``_proxy_ref`` only in the ``__init__`.
Merge remote-tracking branch 'kivy/master' into lang_handler_memory_leak_fix
Merge branch 'lang_handler_memory_leak_fix' of https://github.com/eulerreich/kivy into eulerreich-lang_handler_memory_leak_fix
widget: prefer hasattr instead of try/catch which is a little bit faster