Crash adding articulation to multiple notes

1) Enter two quarter notes
2) Select both via click
3) Double-click staccato or other marking from Articulations palette

Result: crash

Works for range or single selections. Supposedly we make a copy of the list of selected elements and work on that, but apparently it's not really a copy but a reference. As we loop through the elements, when the articulation gets added to the first, it becomes selected, and then when we return to the loop the selection is modified and things go badly from there.

In principle, fix is to make sure we really do make a true copy of the selection at top of Palette::applyPaletteElement(). Since we have a few other related issues of elements not applying properly depending on how they are added, though, we might step back a little and make sure we consider them together.