Improved the next element command

- If nothing is selected, it starts from the begining
- Added all types of segments to the navigation
- fixed glitches with staffs 2-n
- navigation goes automatically to the next staff
- Added handling for all elements represented with spanner segments
- Added handling for elements attached to notes of chords
	modified:   libmscore/navigate.cpp
	modified:   mscore/scoreview.cpp