PopoverMenu: Make :flags a property for use in UI…
…files, or other cases other than calling new_from_model_full(), which generally makes it far easier to experiment with the effect of flags, including by changing the value of the property in the Inspector.
fixes #6030 (comment 1818229)
tested and seems working fine from a UI file, both with/without the property, and toggling it in the Inspector
Edited by Daniel Boles