AC:

+ Implement the multi-file upload with vaadin8 to enable the feature of bulk upload of assets DESIGN-40 e.g. https://vaadin.com/directory/component/multifileupload 

+ Refer the POC from https://jira.magnolia-cms.com/browse/MGNLUI-8376 .

+ Move the code from POC to a feature branch of bulk upload in Dam module.

+ Need to do some refactoring code, focus check styles.

Development notes: 

• The feature development has been done in POC : Drag & drop zone, add assets & discard buttons, grid of status and uploaded assets, separated definition from single upload, JCR save.
• Follow-up tickets:
  • Clear temporal when discarding MGNLDAM-1304.
  • Upload assets in action bar MGNLDAM-1303.
  • Progress bar behaviour MGNLDAM-1292.
  • Single upload behaviour MGNLDAM-1302.
  • Drag & drop area into the grid area investigation MGNLDAM-1297