Use QUrl::toLocalFile() when decoding a string URL

Some percent-decoded characters may still be present in the string URL,
so it is best to use QUrl::toLocalFile() to process the string properly.
This also makes some drive handling "magic" obselete as QUrl already
handles this.

This fixes an issue whereby QML apps residing in local paths requiring
percent-encoded characters (or which import local file URLs with
percent-encoded characters) would fail to load, as the path was passed
to the file system engine without fully decoding the URL.

Change-Id: I8ec2b06f0661e0ac7cc9df79d35ec5cee211f672
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>