No sound in MuseScore 2.0.1 on windows, due to wrong preferences

* Install and use MuseScore 2.0
* MuseScore will store 2 soundfont paths in preferences. One of them point to the MuseScore installation folder.
* Install MuseScore 2.0.1 in another directory than MuseScore 2.0. MuseScore 2.0 will be delete, the soundfont directory and default soundfont too.
* Run MuseScore 2.0.1. Since it shares the same preferences, it will try to load the soundfont from the MuseScore 2.0 folder and so no sound.

We should handle the default soundfont directory in MuseScore, hardcoded, and not save it in preferences or present it to users in Edit > Preferences. We already do this for styles and plugins.