Update WatermarkTask tool

[ilovepdf]

Update WatermarkTask tool

[OlegRa]

1. New 'multi' mode provides way to specify watermark as set of text/image objects, right? Should we check attempt for specifying empty set of items in this mode on client side or it's not an SDK responsibility and this error will be checked on servers side?
2. New nested 'Element' type contains both 'mode' and 'type' fields for specifying same info - text or image element type. Which one is correct?
3. New nested 'Element' type contains two fields which look like logically equal - 'transparency' and 'opacity'. Which one is correct or it's different things and user can specify both?
4. New nested 'Element' type contains 3 new fields 'zoom', 'gravity' and 'border'. All these fields have no comments in PHP SDK and I'm not sure about possible values for them. Please, provide more information about it.

[OlegRa]

1. We should check for non empty elements.
2. type is correct
3. transparency is correct
4. zoom: float value, between 0 and 5; gravity: NorthWest, North, NorthEast, West, Center, East, SouthWest, South, SouthEast; border: int value N >= 0