Suggest nuxeo elements in layouts

PROBLEMS

• People who want to design a custom workflow task layout with something different than date or text can't do it without code
• People who are not satisfied with the default element provided when drag and dropping something in a layout can't do it without code

TODO

• When an element is selected in Designer, offer a dropdown suggestion that allows switching from one element to another

AC

• When an element is selected, a dropdown list appears in the properties panel
• Suggested elements relate to the property's type. e.g. for a string field, I am offered elements like nuxeo-input (default) or nuxeo-textarea but not a nuxeo-datepicker
• When switching from an element to another
  • Rendering is updated
  • Element is still selected so that properties panel still appears
  • If existing properties match for the new element (e.g. previous element had a label and new one too) then property value is kept
  • If properties do no exist for the new element, then their value is lost. Switching back again to the previous element, their value is still forgotten.