As a Studio user, I can select my custom icon

Custom icons appear in the selection drop down list

AC
Custom icons appear in a separate category in the drop down list
Any custom icon uploaded can be selected / filtered in buttons and drawer menu entries screens (not in doctype config)
When no custom icons are available, an indication shows: "start importing your icons"
I can unselect the icon
Svg file display is protected against XSS
When

• nuxeo-operation-button
• nuxeo-search-form
  is used and no icon is selected, a validation warning is shown after save:

  An icon should be set to make the entry visible in Web UI.

  When a custom icon was used and has been deleted elsewhere, Studio displays a validation warning (does not prevent save):

  The icon used for this feature does not exist.