add Window.fullscreen and toggle_fullscreen for sdl2
add borderless property for Window and as a Config option, deprecate fullscreen = 'fake'
make borderless config option and window property work with pygame
remove border debug method
minor cleanup
fix unused variable in _window_sdl2
set borderless to false when fullscreen is switched from fake to auto
Merge pull request #2610 from kivy/sdl2_fullscreen_reset

SDL2 - implement Window fullscreen, borderless and toggle_fullscreen