YITH WooCommerce Pre-Order Premium 2.23.2

Update: Support to WC 3.3.5.
Update: Support to WordPress 4.9.5.
Update: Plugin core.
Fix: Automatic Pre-Order status without managing stock at product level.

Fix: When a Pre-Order product has Sale price but no Pre-Order price, the Regular price was displayed twice.

New: Support to WC 3.3.4.
Update: Plugin core.
Fix: Pre-Order price for variations was hidden when all the variations had the same regular price.
Fix: conflict with YITH Composite Products on Edit product page.
Fix: "Invalid D" message issue on frontend.