Move editor popup handling to CUI and refactor it. Remove unused and less sophisticated menu popup handling in favor of this. The popup implementation is refactored, removing the need for changing the active UI item. This now also correctly handles the case that a popup menu wants to close but has child popup menus open, where previously this would only close the top-most popup. Minor quality-of-life change: items in the image selection popup (PopupSelectImage) in the editor can no...