Fix TextArea's font
Fix TextArea's font

Using a Binding element overrides TextArea::font with a value from
TextAreaStyle::font, whereas the style is only supposed to provide
the default value. Use TextSingleton's font to avoid the original
binding loop.

Task-number: QTBUG-39328
Change-Id: I90113ef03dd31b2a060dfe12361708f99b35eb53
Reviewed-by: Liang Qi <0f3d3d9587f4f01263b2640b7eef7a92a160ec94@digia.com>
Reviewed-by: Caroline Chao <cb6a0059f4e516e853a34cb558c25ca5a9bcf4ab@digia.com>

mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,mizukusak/qtquickcontrols