Manipulate bytes instead of characters

Summary:
D21155 trucates a QString. Since most Linux FS specify filename limits in bytes, convert the url to
QByteArray and then truncate.

As for truncating a UTF-8 sequence in middle, will it have any noticeable impact?
After truncating, the url loses any meaning it had initially. So IMO a few characters
at the end which were initially not a part of the url don't make any difference.

Reviewers: bruns, dfaure, thiago

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21317