Fix available operators for a generic search widget

Steps to reproduce:

• on a content view search screen, add a "generic search" widget type
• bind it to field ecm:path and widget type select_path_tree, chosing operator "STARTSWITH"
• save the popup
• open the edit popup again
  => only the "IN" operator is available

(note that same problem cannot be reproduced when chosing target platform "5.6" where select_path_tree widget type is unknown -> maybe this widget type is misconfigured)