Allow selecting attachments from inactive skins in the viewport

Currently, attachments in inactive skins can be displayed in the viewport, but they cannot be selected there. This behavior is generally necessary to avoid confusion, but there are cases where it would be useful to temporarily allow selection while setting up skins.

For example, when creating a bone, you can hold `Ctrl` and select attachments to make them children of that bone. However, if attachments that are intended to be children of the same bone belong to different skins, they cannot all be selected together in the viewport.

For instance, suppose you are creating an `arm` bone and want an `arm` attachment containing the base skin, a `sleeve` attachment, and a `bracelet` attachment to all be children of that bone. If each of those attachments belongs to a different skin, you can only make one of them a child this way, and the others need to be parented using another method.

It is possible to work around this by using the `[bone]` tag in the PSD so that all of those attachments are assigned to the `arm` bone in advance. However, it would still be convenient to have a way to temporarily allow attachments from inactive skins to be selected in the viewport.

**Related forum thread:**
https://esotericsoftware.com/forum/d/30587-disable-skin-selection-for-rigging-purposes/

Start in the viewport selection behavior for attachments and trace how inactive skins are excluded. The change is done when users can temporarily select attachments from inactive skins for rigging, without changing the normal behavior that prevents confusion.

Written by the indexing model from the issue text.