* Fix for #26261. Changes CupertinoTextField's cursorColor to read from CupertinoTheme instead of prior default of activeBlue. CursorColor will still default to activeBlue for light theme and activeOrange for dark theme if a primary color has not been specified for the CupertinoTheme.
* Reverted unnecessary changes in XCode file.
* Updated text_field.dart per suggestions from @gspencergoog
* Updated comments for cursorColor to reflect appropriate hyperlinks per @Hixie
* Simplified cursorColor assignment per @xster
* Added test in cupertino/text_field_test.dart to check for correct cursorColor based on CupertinoTheme per @Hixie & @xster.