fix #21247: Change scrollbar policy of navigator

[cardinot]

Small change, just to hide the horizontal scrollbar when it's not necessary.
http://musescore.org/en/node/21247

[lasconic]

Unfortunately it's not that a small change. Last time we tried this, we encountered all sort of problem. If you resize the navigator slowly until the scrollbar is hidden and back again and do it often, you can end up in a redraw loop... If you can't reproduce this behavior, I will test it again, but I think it was a problem

[Jojo-Schmitz]

See 9f940a0, which is where it got changed last, as seen with 'git blame -L47,47 mscore/navigator.cpp'

[lasconic]

See http://musescore.org/en/node/21247#comment-97700 Please reopen if you find a proper solution