Support specifying fallbacks in font request on QPA

Support specifying fallbacks in font request on QPA

Because the QPA font database would query fallback families inside
findFont(), support for requesting multiple font families in order
of preference (like QFont("Times New Roman, Arial")) did not work,
because the Arial fallback was never attempted. To fix this, we
pass in the queried fallbacks and make sure they are tried before
any platform specific fallbacks.

Task-number: QTBUG-20986
Change-Id: Idb2b717856f013ce2874f00a8debaff60176d2fc
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>