Add font family alias resolving to QPA

Add font family alias resolving to QPA

By default it doesn't do anything special, but for fontconfig font db
it will do some tricks to convert alias like serif to actualy font
family names like Times.

Change-Id: I73058d187a5422e6f93bc1b728cecdc0250d5e1e
Reviewed-on: http://codereview.qt-project.org/5984
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>