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.
Check comment, double words.  
Add a comment for these 3 functions to explain they are used for navigation here in the .h
Move the comment on the function into the header comment of each function in the .cpp