Add `mountpoint` option to iconSet

[schloram]

There is also an mountpoint variant of a page (if content_from_pid is set). We should consider to make this editable with a mountpointIcon option.

Example:

dokType: 87
label: 'My Custom pageType'
iconSet:
  defaultIcon:
    # 'source' or 'identifier'
  hideInMenuIcon:
    # 'source' or 'identifier'
  rootPageIcon:
    # 'source' or 'identifier'
  mountpointIcon:
    # 'source' or 'identifier'