Allow pasting copied bones to multiple selected mesh attachments in Weights view

Currently, after copying bones from the list of bones bound to a mesh in the Weights view, pasting is only possible when a single mesh attachment is selected. It would be useful if pasting were also supported when multiple mesh attachments are selected simultaneously.

**Related forum thread:**
https://esotericsoftware.com/forum/d/30212-spine-43-released/9