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 <liang.qi@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
TextField::font: fix the default value handling

Assigning undefined to font is not possible, so use TextSingleton::font
as a fallback instead of undefined.

Change-Id: If148f7773ca784c0a1eb78f952dfad073fa9b1e3
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>