use dconf to detect current icon theme

GNOME and derivatives ( Ubuntu, elementaryOS ) have switched to dconf over gconf to manage the settings.
Regarding detecting the system icon theme Qt libraries still try to access it via the old gconf ( that is fine on some system but don't work on other ).

So this commit try to use dconf to retrieve the name of the current icon theme ( and apply it accordingly ). In case of failure the default behavior is used.

KDE is well covered by default behavior.