Add 2x icons for retina displays.

This change updates the mode selector/fancyactionbar
in Qt Creator with new high-resolution icons.

Set Qt::AA_UseHighDpiImages on Qt 5.1 and higher to
make QIcon generate high-dpi pixmaps. Add "@2x" mode
images.

Update StyleHelper::drawIconWithShadow to handle high-dpi
pixmaps. The shadow-drawing algorithm is kept in device
pixels.

Change-Id: I411b7a24e534a2d75a1bbdc4d10219dcbea26bed
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>