Convert GIO icons to QIcon

Added a new method to Kommodity::Icon in order to convert icons to
QIcon.

This feature is needed by some classes such as PlacesModel.
Recognize icon type and convert to QIcon accordingly

If we are dealing with a GFileIcon, g_icon_to_string() might return
a path. If that's the case we need to tell QIcon, otherwise we assume
it's an icon name but if the current theme doesn't have it we load
the "unknown" icon.

See also https://developer.gnome.org/gio/2.32/GIcon.html#g-icon-to-string
Recognize icon type and convert to QIcon accordingly