Fix StaffType::operator==

For TAB_STAFF_GROUP, the equality operator does not check all the member variables (does not call isSameStructure())

Also, simplified some duplicate tests.

This also fix the bug of the Staff type editor not completely updating neither the preview or the score staff on (some?) parameter changes.

(Re-layout of initial clef and of time signatures on staff group change is not fixed: these are old bugs, caused by 'eccessive' optimization of the layout process).