Fix possible divide by zero in QPlainTextEdit

If the font engine for some reason fails to get font metrics
for the font, the application should still not crash.

[ChangeLog][Widgets][QPlainTextEdit] Fixed a possible divide
by zero crash when font metrics were missing for the font.

Task-number: QTBUG-40347
Change-Id: I571bc3eace07cdbee6f9ce9aa649df95412aed71
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
QMacStyle: Use NSView rendering for some inactive widgets on 10.10

Right now, we use them for inactive non-editable combo box, check
box, and radio button only on Yosemite. We keep as much as possible
the previous behavior on older versions.

In addition, we add a way for QQuickStyleItem to specify the window
the item is on. This is currently without effect, since we don't
seem to take the inactive window state into account.

Task-number: QTBUG-40833
Change-Id: I2fb2a99e6adf1972f881195b79b07ce85a960273
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>