Having icons of all different kinds of sizes makes it difficult to get
the painting right in all cases. It definitely fails when the szsytem is
running in multi-monitor mode with different scaling factors. Unifying
the dimensions makes it possible.

This change should not cause any visual difference in a purely scaled or
unscaled screen.

Task-number: QTCREATORBUG-18869
Change-Id: I3a864c9a86036b4f4f3b350a489077b247ca85b7
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>