Add support for task variables on nuxeo-dropzone

The following changes on  nuxeo-dropzone API occured: Usage of  document and  xpath properties was dropped and from now on the  value property should be used; blobList  property is now called  multiple ; A new property called  valueKey was added and it represents the key used on the JSON object to store each entry, when using multiple mode. Examples: The file schema uses  files:files to store blobs, each blob being stored in  file key. On the other hand, when using the  blobList **  type defined in Studio, each blob would be stored in  item key. updateDocument property was removed which means that the  nuxeo-dropzone element won't automatically update the document. From now it, the element using  nuxeo-dropzone is responsible to trigger an update call if needed. The unique default case ( nuxeo-document-attachments ) was already covered and can be seen as an example of what can be done. Related to these changes, some refactoring was done. Due to that the  S3 Upload functionality may need to be further tested in order to understand if no regressions were introduced (see NXP-24979 , ELEMENTS-845 , NXP-26242 ); The  WOPI addon was also considered and should be working properly accordingly to  NXP-26242 ;