Expose "Select Instrument" in Inspector for Instrument Change text

Currently mid-staff instrument changes are very non-obvious. After adding the special text element, users may find themselves mystified, as Tantacrul did in his video review (watch from https://youtu.be/4hZxo96x48A?t=1149 for a very clear explanation of how bad this is). It may be a little much to eliminate the "Select Instrument" dialog and put its contents, complete with tree view and search field, in the Inspector, but a button in the Inspector to access it is a must.