[Merge] lp:~justinmcp/qtubuntu-media/metadata into lp:qtubuntu-media

[Justin McPherson]

This patch came out of work to have Oxide use an "external" player for media playback, which used the QML player backed by qtubuntu-media.

At the time this patch was written, qtubuntu-media would only provide metadata when it could be found by interfacing with the mediascanner - which scans local content for information.

>From the perspective of playback in Oxide; content provided for playback was never local - so the resolution information was never available, but it is necessary for a working Oxide solution.

Additionally the patch contains; 
- An optimisation, in the form of first checking if the Url for scanning represents a local file - it saves a large amount of calls and a throwing/handling of an exception if the simple test is done first.
- A call to .path() of QUrl instead of .toString(QUrl::RemoveScheme).toStdString() which can return results that are not usable by the .lookup() method of the store.

If the changes are still relevant I am happy to port them, or I can just drop this proposal.
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https://code.launchpad.net/~justinmcp/qtubuntu-media/metadata/+merge/260679
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