An attempt to fix icon theme in file dialog

Currently - there is the problem with the color of icons in FileDialog, on my dar theme they are dark too. Not sure how to handle it the best way, so I'll try to do at as many times as it needs The first solution is about #if !defined(Q_OS_LINUX) && !defined(Q_OS_UNIX) flags, this is made assuming that the system on theese platforms already has in mind what icon theme it's going to use by default, and there is no need to override it