Allow dont_care value for window position

[HurricanKai]

Allow parsing null as window position, and pass the respective dont_care value to the backend.

[Perksey]

Requires API changes to change certain properties to nullable, while this shouldn't be source-breaking I think this should be for 3.0.

[Perksey]

#2278 closes this but we need to remember to document it before closing.