lang: new dynamic classes, that deprecated templates.
lang: avoid usage of Clear instruction specific to kv language, instead, introduce a "-" for avoiding to apply any previous rules. And use it for Button and ToggleButton. Benchmark show improvement of 11.10s to 8.77s for 10000 creation.
lang: use a specific cython class for storing all the data of a properties, instead of a dict. (Benchmark on the macbook reduced from 38.39 to 36.53. Note that the benchmark bench only the widget creation, and not really animation / layout / etc. So properties are not really stressed.)
events: add __events__ discovery, no need to register_event_type() anymore.
merge master