FormElement should be easier to change big-icon (in create Block List) and small-icon (in ToolBar)

Description

Currently, it's hard to change the form element big icon in Block browser. Since form elements inherit from block data, we should allow the use of the ImageUrl attribute to specify the icon of form elements, like other block types.
For the small icon in Form Element widget, it's easy to change the default one by overriding the css class, for example:
.epi-forms-icon.epi-forms-icon--small.epi-forms-mycustomelementblock__icon