[A11y] Add LABEL_FOR/LABELLED_BY AtkRelation pair between instrument and panel

[Joanmarie]

When a project involves multiple instruments, there are multiple buttons (disable, mute, activate solo, instrument effects) with the same accessible name and multiple events with the same (or very similar) name(s). Without an accessible relation set between the instrument label and the container holding these objects, it can be difficult for a user who is blind to identify the instrument with which a given button or audio event is associated.

This issue should be solvable by adding a LABEL_FOR/LABELLED_BY AtkRelation pair between the instrument label [screenshot item 1] and the container which holds the aforementioned objects [screenshot item 2]. You should be able to verify that the relationship has been set in Accerciser's Interface Viewer [screenshot item 3].

With such a change, Orca will be able to indicate the instrument when a user initially tabs into it, and also provide that instrument as part of the context when the user uses Orca's WhereAmI command. (Note: this should JustWork(tm) with the current Orca, but it might require a tweak in a custom script for Jokosher.)