Split functions.php into smaller files

[crftwrk]

This PR cleans up the patchwork mess in functions.php and splits functions into smaller files in inc folder. functions.php uses

require_once('inc/breadcrumb.php');
...

to include them.

Files have now deny direct access, which we should add to all php files step-by-step:

defined( 'ABSPATH' ) || exit;

• Comments have been updated and follows the standard
• Files have now a version to track each file changes

Tested and works fine. If this PR is accepted and merged, I'll do the same with wc-functions.php.

@justinkruit if you like it, merge it!

[justinkruit]

Looks good!

Also, is there a reason we use the if() : endif; instead of {}?