Improve interaction between QPlatformTheme and QPlatformIntegration hinting.

Ensure that custom style hints returned by subclassed
QPlatformIntegration::styleHints() are used if not explicitly overridden
by subclassed QPlatformTheme::themHints().

Task-number: QTBUG-34589
Change-Id: I0a3114cb00b532d2a5cd40941eca192b32412bcf
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Allow the platform to specify a press and hold delay.

Press and hold is an interaction available on many platforms,
particularly when touch is involved. In Qt Quick this is exposed to the
user via MouseArea::onPressAndHold. This value should not be hard-coded,
but rather use a platform-specified default. This commit adds the
low-level hooks necessary for that to happen.

Task-number: QTBUG-24793
Change-Id: I621a8ac9de66b881e34336228056bffbb6306a70
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>