sdl2: start to write a new window provider based on sdl2. sdl2.pxi has been created from the ignifuga game engine, but maybe functions and definitions are missing. Need to create a generator out of it. At some points, window_sdl (used currently only on iOS) should be removed.)
sdl2: add missing window_sdl2 files, and correctly implement 'auto' fullscreen. read all the missing window event too.
sdl2: add sdl2 image core provider (not complete, need more conversion format, and save support)
sdl2: support for text rendering
sdl2: ignore generated .c
sdl2: fix ignore
sdl2/text: fix color bgra -> rgba
core:sdl2 clipboard provider
- fixed conditional import for pep8 checker
- moved the import to a conditional python 3 import instead of a global
  one, since that seems to be the original intent. Addresses #1900
Merge branch 'master' of https://github.com/kivy/kivy
Merge remote-tracking branch 'upstream/master'
Change image pygmae to use file handles instead of path because of unicode issues.
Change texture cache key to keep str type of the filename so we don't get encoding issues in automatic encoding.
- convert data to a bytearray to more consistently get color info accross
  python versions
add debug method for recursive updates
Use wrap to give correct name to mainthread wrapped func. Fixes #2027.
fix nested scrollviews and add passoff functionality
simple keyboard implementation for rpi (incomplete, some key missings)
rpi-keyboard, fixes spacebar/tab and other key usages

incidentally fixes keycodes in core/window/__init__, a lot of missing/wrong things