Allow Ctrl+Left Click multi selection of mesh attachment timelines in the Dopesheet

Currently, in the Dopesheet view, Ctrl+Left Clicking the name column of a slot attachment timeline allows multiple slots to be selected.

However, the same is not possible for mesh deformation timelines. Ctrl+Left Clicking the name column only selects a single mesh attachment.

Allowing multiple mesh attachments with deform keys to be selected directly from the Dopesheet would make it easier to edit multiple mesh deformations together, without having to switch back to the hierarchy to update the selection.

This would be particularly useful when the mesh attachments are located under different bones or in different parts of the hierarchy.

**Related forum thread:**
https://esotericsoftware.com/forum/d/30406-%E6%9C%89%E5%85%B3%E6%91%84%E5%BD%B1%E8%A1%A8%E5%9B%BE%E8%A1%A8%E7%9B%B8%E5%85%B3%E7%89%B9%E6%80%A7%E5%92%8C%E5%8A%9F%E8%83%BD%E5%BB%BA%E8%AE%AE

Start in the Dopesheet view at the name column handling for slot attachment timelines, then compare it with mesh deformation timelines. Verify the existing hierarchy selection behavior and the related forum thread for context. Done means Ctrl+Left Click can select multiple mesh attachments with deform keys directly in the Dopesheet, including attachments under different bones.

Written by the indexing model from the issue text.