Open files in TagLib extractor readonly

Summary:
This is useful if the extractor runs sandboxed, and read-write file access is denied.
TagLib would retry it readonly, but this saves one syscall.

Test Plan:

• Autotests run fine
• Tags are still indexed, tested with "baloosearch artist=..." and "baloosearch genre=..."

Reviewers: Frameworks, vhanda, cgiboudeaux, dfaure, mgallien

Reviewed By: mgallien

Subscribers: mgallien, ngraham, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D8330