Qt Designer: Develop broader support for rearranging tree in Object Inspector

Drag-and-drop functionality for inserting an existing object node as a child of another in the Object Inspector was requested in QTBUG-108917.

Several further enhancements would facilitate manipulation of the tree structure, as follows:

• Drag-and-drop operations for changing order of sibling nodes, as well as for inserting a node arbitrarily between any adjacent pair, or before the first child of a node or after the last.
• Buttons for promoting and demoting a node within the hierarchy, and for moving it higher or lower within a sequence of siblings.