Add a "Move Slot" mode for quickly repositioning slots in the draw order

Currently, there are two primary ways to move an existing slot to a different position in the draw order:

1. Drag and drop it in the tree view (either in the Draw Order section or with only the Slots filter enabled).
2. Use the draw order hotkeys.

However, when working with very large skeletons containing many slots, both approaches can become time consuming because the tree view may be extremely long.

It could be useful to have a workflow similar to `Set Parent`. After activating a `Move Slot` mode, the user could click a target slot to specify the destination, and the currently selected slot or slots would be moved immediately above or below that target slot.

This would make it much easier to reorder slots precisely without having to drag them through a long tree view or repeatedly use the draw order hotkeys.

**Related forum thread:**
https://esotericsoftware.com/forum/d/30381-%E8%AF%B7%E6%B1%82%E6%BB%91%E5%9D%97%E7%BA%A6%E6%9D%9F%E7%9A%84%E5%8E%9F%E5%8A%A8%E7%94%BB%E5%B8%8C%E6%9C%9B%E8%83%BD%E8%B0%83%E6%95%B4%E5%85%B3%E9%94%AE%E5%B8%A7%E6%95%B0%E5%80%BC/12

Start by reviewing the existing Set Parent workflow, then compare how draw-order changes work in the tree view and through hotkeys. The proposed mode should let users activate Move Slot, select a target slot, and move the selected slot or slots immediately above or below it; verify behavior with large skeletons and multi-selection.

Written by the indexing model from the issue text.