Breaking: #77345 - EXT:form - Remove deprecated IMAGEBUTTON element 

See forge#77345

Description 

The IMAGEBUTTON element has been removed in TYPO3 v8.

Impact 

Using the IMAGEBUTTON element is not working anymore, i.e. no IMAGEBUTTON element will be rendered.

Affected Installations 

All installations using the IMAGEBUTTON element.

Migration 

Remove all usages of the IMAGEBUTTON element.