[Tracker] Snif files in MIME filtered windows asynchronously

[nephele]

This is so applications that filter their open... actions with mimetypes can allow users to open stuff that was obtained without setting a mimetype, a good example i encountered was png files synched over git which then didn't have any mimetype and thus weren't visible to WonderBrush.

Proposed solution: If tracker stumbles upon a file with no mimetype set during it's filtering add that to a list of to be-sniffer files and start sniffing asynchronously (and add to the list based on the result)

Alternatively Tracker could do this for all it's windows and not just during filtering.

[humdinger]

In case it wasn't clear, the above described the experience in WonderBrush's open file dialog.

[jessicah]

I think I would prefer the alternative scenario, maybe an option to disable auto-sniff?

[pulkomandy]

John's patch for thumbnails sets up the required code to run a task in the background and scan all the files. I think we could just modify it so it identifies the files in the process?