add w3-padding

Author: tekkimariani

src/w3css/W3css.php

@@ -306,6 +306,7 @@ class W3css{
     public const ROUND_XXLARGE = 'w3-round-xxlarge ';
     public const PADDING_SMALL = 'w3-padding-small ';
     public const PADDING_LARGE = 'w3-padding-large ';
+    public const PADDING = 'w3-padding ';
     public const PADDING_16 = 'w3-padding-16 ';
     public const PADDING_24 = 'w3-padding-24 ';
     public const PADDING_32 = 'w3-padding-32 ';