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 remote-tracking branch 'kivy/master'
Update animation.py
doc: revisions to input/providers/mouse.py
doc: revisions to input/providers/wm_pen.py
doc: revisions to input/providers/wm_touch.py
screenmanager: reduce transition duration, and set the default transition to SlideTransition.
dropdown: delay the container binding, in order to use the dropdown in kv.
actionbar: expose ActionToggleButton in the documentation, and share the same style as ActionButton.
style: dpi fixes for spinner and slider.
showcase: new showcase design. few bugs are still here, but mostly working.
uix:TextInput add `minimum_height`, `line_spacing`,
make sure changes in password change the Text display.
Merge pull request #1439 from kivy/textinput_fixes

uix:TextInput add `minimum_height`, `line_spacing`,
Update animation.py
Update animation.py
scrollview: if there is nothing to scroll, avoid touch processing.
Factory_registers add ButtonBehaviors so that it can be used in
kvlanguage
screenmanager: fix shader-based transition when they are not used in fullscreen. Now it works perfectly even when used in a specific pos/size.
screenmanager: add a new ScreenManager.switch_to method to add the screen into children, set transition options, do the switch, and remove the previous screen. It also handle name conflict. closes #573, #1045