Finished next/prev element command. Added home/end command.

Fixed behavior for system attached elements.
Changed name
Added prev element command.
Added home/end command and shortcuts for them
	modified:   libmscore/navigate.cpp
	modified:   libmscore/score.h
	modified:   mscore/actions.cpp
	modified:   mscore/data/shortcuts.xml
	modified:   mscore/scoreview.cpp