Add Window position manipulation

[KeyWeeUsr]

ref #3799

from kivy.app import App
from kivy.clock import Clock
from kivy.core.window import Window


class TestApp(App):
    def show(self, dt):
        pos = Window._get_window_pos()
        Window._set_window_pos(*[p + 1 for p in pos])

    def build(self):
        Clock.schedule_interval(self.show, 0)
TestApp().run()

For now getter and setter only. Not sure if it needs to be a separate thing from Window.top for example, or if it needs to update those two properties.

@dessant @matham ? 😋

[dessant]

@KeyWeeUsr, the existing Window coordinate properties could just have the correct values, and documenting the differences of this coordinate space.

EDIT: that could also create issues (#3766), i'm not sure. 🔮

[KeyWeeUsr]

Yeah, well... it's SDL2 property to be specific, but if it's necessary, we could have additional kivy property that would be just inverted top or left or something.

Edit: We could have just a note that it's a matter of window creation, where the [0, 0] is at top left as it's obvious from the property names. That shouldn't be really an issue unless there is a single property named pos that would bring confusion.