Screen attached property: expose pixelDensity property

Because logicalPixelDensity is different than physical density on
multiple platforms, having the actual density is critical to be able
to design resolution-independent controls.

Change-Id: I06bbdc6e6869718058a796ca737668ce69802f2b
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
QtQuick.Dialogs: resolution-independent controls use actual resolution

logicalPixelDensity ended up hard-coded on Android, which made the
buttons and the FileDialog too small again.

Change-Id: Ic1805246c4466d7167d422ba48ae5ec37978d332
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>