Next element command

Can be merged by itself, but needs integration with accessible_toolbar.

Adds four new actions:
next-element
prev-element
home
end

In combination with score_accessibility branch will be how blind users will read scores.
we don't use inherits on Score elements. It's not guaranteed they will keep on being QObject so we avoid depending on this.

Eventually, you could use dynamic_cast or code isSpanner in element.h/.cpp. I will check with Werner to see if he has a preference.