Fix superview issue on iOS

[AdityaPahilwani]

Fixes #192

current the lib access UIWindow access by
UIWindow *keyWindow = [UIApplication sharedApplication].keyWindow;
and was deprecated in ios 15 and can return nil https://developer.apple.com/forums/thread/695932
and when I debugged this and passed
UIWindow *keyWindow = nil;
it threw the exact error
Unable to parse constraint format: Unable to interpret '|' character, because the related view doesn't have a superview V:[self(>=48)]| ^

By accessing UIWindow by
UIWindow *keyWindow = [[UIApplication sharedApplication] delegate].window;
which would point to the window of AppDelegagte.

We didn't saw any crash reports after this fix.

[savv]

@cooperka, many thanks for this library! What about merging this and making a new release?

[cooperka]

thanks for the fix and documentation, @AdityaPahilwani! i will merge and release as a minor (rather than patch) version bump since i have not been able to verify whether or not this change may break earlier versions. it likely does not.