[Trunk] Changing icon size has different effect between empty state and open score

1. Have no score open.
2. Open Preferences.
3. Change horizontal icon size to 40.
4. Open score.

Expected result: The size is the same.
Actual result: The toolbars have expanded.

Discussion: Will contract or expand (randomly I think) if you have a score open when changing and closing (and perhaps re-opening).

Using MuseScore 2.0 Nightly Build (4300) - Mac 10.4.11.