Document the multi property editor feature

Description

This feature lets the user open multiple property editors at the same time. The user can lock each property editor so that the subject node does not change by each selection.

Test

• Extra Property Editor Manager should be enabled
• From menu/view open Properties
• There should be a + Button on top, beside the lock button.
• By pressing it, it should create an extra property editor.
• By closing the tab, it should be closed.