Support html5 upload features

[Guite]

Upload fields should get a new property for defining whether they are asynchronous (for using modern upload technologies based on html5).
Initial generator implementation should support the following use cases:

• Allow only images if using image fields (mime filter)
• Option for client-side filesize checks
• Option for client-side image preview
• Option for having a drop zone beside input field for desktop file drag-ins
• Option for progress bar feedback

Multiple uploads are not treated yet, but should be supported in the design as they are planned to be included in #123.

• https://www.sitepoint.com/html5-ajax-file-upload/
• http://blog-autoren.com/2012_faq-wordpress-bilder-per-drag-and-drop-einfuegen/210/
• Github style simple add image to post. zikula-modules/DizkusModule#204