Default themes backwards compatibility message

[ramiy]

Since "Twenty Thirteen", the default themes are not meant to be backward compatible. The newer theme relies on new functions and markup introduced in latest wordpress version.

This is why each theme has backwards compatibility functionality in inc/back-compat.php file that adds upgrade notice, customizer notice and preview notice when the user is using unsupported wordpress versions.

This patch improves a bit the code, moving the message into a separate function, to be used in different places.

See the attached patches. Each patch for a different theme.