CMS-873 Content Manager: Make Content Form draw components based revised logic

FormGenerator contains current logic for generating ExtJS components based on Content
  the file has currently two functions, one for "new content" and another for "existing content".
Note: FormItemSet also uses FormGenerator to draw it's children.

ContentDataPanel.buildContentData generates data to be submit on save

Please feel free to change refactor to best adopt to revised logic:

http://wiki.enonic.com/display/dev/WEM+Content+Form+logic

Move handling multiple occurrence to mixin so it could be widely reusable