improve callback validation for data fetching

[afonsolpjr]

briefly:

• Replace complex condition with is_callable
• Ensure private/protected methods are handled correctly, since it was not even calling class methods.

long:

The Class" Redux_WordPress_Data" in '\redux-framework\redux-core\inc\classes\class-redux-wordpress-data.php', which populate some fields, is not calling class methods/callable arrays when its type is set to 'callback' and the 'args' value is the callable array.

on '\redux-framework\redux-core\inc\classes\class-redux-wordpress-data.php' , on the switch($type) statement, line 473, we have:

case 'callback':
					if ( ! empty( $args ) && is_string( $args ) && function_exists( $args ) ) {
						$data = call_user_func( $args, $current_value );
					}

					break;

which conditions does not include callable arrays, like the ones mentioned in Example 2 in this php doc

I don't know if originally it shouldn't call methods/arrays as custom callbacks, but since it is common to use callbacks in this format, it can be useful to include this.

[kprovance]

I vaguely recall this being necessary some years ago because a user somehow passed the arg as something other than a string, causing a crash. It was foolish to assume theme devs would use common sense and maybe read the docs. That was my bad and thus the workaround. I don't recall if we used is_callable before, but I'm sure it's in the git history.

I'll check into it and see just to ensure we don;t go backward.