Add sdl2 system cursors

[Bakterija]

Hello.
I have tested kivy with sdl2 system cursors on windows, ubuntu, android 4.4 and everything works very nicely. I would like to share it with other people.

This commit adds necessary structs from SDL2 and a set_system_cursor method for kivy window where you can replace your app cursor.

[KeyWeeUsr]

Seems nice! If no one shows for testing, I'll look into it soon-ish.

[KeyWeeUsr]

Everything except wait_arrow works just fine. Does it work for you? Share a screenshot, please.

[KeyWeeUsr]

Run the docstring e.g. through http://rst.ninjs.org and fix the formatting, please :)

[Bakterija]

I am not sure what you want to fix, it looks like other kivy docstrings.

[Bakterija]

Wait arrow is a crosshair for me. It depends on platform.

[KeyWeeUsr]

Yes, it depends on a platform, but neither of those should return a crosshair. See windows, wayland, cocoa/mac, x11, docs :)

I think you ran an example like this:

from kivy.app import runTouchApp
from kivy.lang import Builder
runTouchApp(Builder.load_string('''
#:import win kivy.core.window.Window

GridLayout:
    cols: 4
    Button:
        text:  'arrow'
        on_release: win.set_system_cursor('arrow')
    Button:
        text:  'ibeam'
        on_release: win.set_system_cursor('ibeam')
    Button:
        text: 'wait'
        on_release: win.set_system_cursor('wait')
    Button:
        text: 'crosshair'
        on_release: win.set_system_cursor('crosshair')
    Button:
        text: 'wait_arrow'
        on_release: win.set_system_cursor('wait_arrow')
    Button:
        text: 'size_nwse'
        on_release: win.set_system_cursor('size_nwse')
    Button:
        text: 'size_nesw'
        on_release: win.set_system_cursor('size_nesw')
    Button:
        text: 'size_we'
        on_release: win.set_system_cursor('size_we')
    Button:
        text: 'size_ns'
        on_release: win.set_system_cursor('size_ns')
    Button:
        text: 'size_all'
        on_release: win.set_system_cursor('size_all')
    Button:
        text: 'hand'
        on_release: win.set_system_cursor('hand')
    Button:
        text: 'no'
        on_release: win.set_system_cursor('no')
'''))

and changed the icons one after another. Try to press only the wait_arrow button and nothing else and see if it even changes the icon. It doesn't for me on windows and that one uses the latest stable SDL2, which probably means it's an upstream bug, I guess.

If you have the same result, it might be a good idea to include a note about that.