Utils: Helper function for dpi aware pixmap loading

QIcon can load a @2x variant of an image in case it exists and
in case the devicePixelRatio is 2. In Qt Creator, we need the same
functionality for loading QPixmaps. This patch adds the helper
function.

Change-Id: I8e50d1dabb7c8dce1347c8e0a0ce611ec102f9d3
Reviewed-by: Orgad Shaneh <orgads@gmail.com>