Ensure that QUrl::{to,from}LocalPath encode/decode properly

Ensure that QUrl::{to,from}LocalPath encode/decode properly

Unlike path(), toLocalFile() isn't reporting a URL component, so it
should decode the percent-encoded characters fully. This extra
decoding pass is meant to catch %00 to %1F, %7F and %25 (the percent
sign itself).

It also catches %80 to %FF, which aren't decoded because they don't
form UTF-8 sequences. That means QUrl::toLocalFile() has undefined
behaviour if the path contained non-UTF8 sequences.

Task-number: QTBUG-25459
Change-Id: Iab5a0ba6afcfc4510e297984f2ffc208cedd752b
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>