QUrl: fromLocalFile(QString()) should lead to an empty URL.

This is much more useful than the URL "file:", it allows to use
"empty path" and "empty URL" for the same meaning (e.g. not set).

QFileDialog actually uses "file:" though, as the URL for the
"My Computer" item in the sidebar. This patch preserves that.

[ChangeLog][QtCore][QUrl] QUrl::fromLocalFile now returns an empty URL
if the input string is empty.

Change-Id: Ib5ce1a3cdf5f229368e5bcd83c62c1d1ac9f8a17
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Windows: Use common code paths for default font families.

Factor out function to determine the font family for a style
hint and use that for the FreeType font database as well.

Task-number: QTBUG-39961
Change-Id: Ic5ec0928c7994b61e4d2b10d829826658b1aa1d5
Reviewed-by: Björn Breitmeyer <bjoern.breitmeyer@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>