Enable override of default button icons via bundle contribution

Attempting to override the icon attribute of buttons whose icons are computed/hardcoded (e.g. <nuxeo-delete-document-button>) via bundle contribution results in no effect. While default document actions' buttons are accessible for editing through Studio, assigning a new value to their icon property results in no effect.

Additionally, certain buttons such as <nuxeo-document-edit-button> provide no icon attribute for override, resulting in a lack of look & feel customization.

EXAMPLE:
1. In Studio Designer > UI > Buttons, select deleteDocumentAction for editing
2. Under Element Binding > Attributes, assign a new value to the icon attribute and Save the Studio project
3. After loading the Studio project/refreshing the Nuxeo instance, the <nuxeo-delete-document-button> default icon persists.