Fix 21190 - misplaced ledger lines for whole notes

Fixed by computing a stem x-position in Chord::layoutPitched() which corresponds to the actual chord note widths and positions, rather than relying on a global, fixed value (score()->noteHeadWidth() ).
Improved by scanning all chord notes and factoring some common computations.
Merge branch 'Fix_21190_bis' into Fix_21190_whole_notes_ledger_lines

Conflicts:
	libmscore/chord.cpp
Merge pull request #428 from mgavioli/Fix_21190_whole_notes_ledger_lines

Fix 21190 - misplaced ledger lines for whole notes