Deprecated _showxv

[radarhere]

My attempt to deprecate Image._showxv, without raising a DeprecationWarning when calling Image.show or Image._show.

Inspired by #4693 (comment)

[hugovk]

Would something like this be simpler?

 def _show(image, **options):
-    _showxv(image, **options)
+    from . import ImageShow
+
+    ImageShow.show(image, **options)


 def _showxv(image, title=None, **options):
     from . import ImageShow

+    warnings.warn(
+        "_showxv is deprecated and will be removed in a future release. "
+        "Use Image.show instead.",
+        DeprecationWarning,
+        stacklevel=2,
+    )
     ImageShow.show(image, title, **options)

Also adds stacklevel=2 for the warning to show the line of the user's own script calling _showxv (where action is required), instead of the line in Image.py.

[radarhere]

I'm just thinking that if a user has overridden _showxv and expects their custom function to be called by im.show, changing the code as you have suggested would break their code, instead of showing a deprecation warning.

[hugovk]

Thanks!