Fix #320126: Grace note slash layout

This will fix 2 bugs - grace notes slashes will now show up in TAB-COMMON and the slashes will no longer be stranded above the staff when switching between TAB-COMMON and TAB-FULL.

Backport of musescore#8137 / musescore#8106 (both closed meanwhile) to 3.x

libmscore/chord.cpp

@@ -2304,6 +2304,8 @@ void Chord::layoutTablature()
                               p.rx() -= _stem->width();
                               }
                         _hook->setPos(p);
+                        if (_stemSlash)
+                              _stemSlash->layout();
                         }
                   }
             }