Tremolo bar appears above wrong staff

This affects 2.0.3 and later versions.

1) new score with two staves
2) add tremolo bar to any note or rest in bottom staff

Result: tremolo bar appears above top staff

Apparently this was was a bug fixed during the beta period for 2.0 - #35136: Tremolo Bar displayed over incorrect staff, crash on delete. However, it was broken after the release of 2.0.2 - the fix applied in https://github.com/musescore/MuseScore/pull/1374/files was reverted in https://github.com/musescore/MuseScore/commit/c96c78fbc6fed79e060d0a31c…. I'm not sure what drag & drop issue needed fixing, exactly, but it seems adding ON_STAFF back to the element flags for tremolo bar fixes the issue at hand and drag & drop still works as far as I can tell.

Unless someone can think of a reaosn not to add ON_STAFF back, I propose doing that, and will submit a PR soon.