[Mount ISO Action] Refactor to direct async DBus calls

Kai Uwe Broulik requested to merge work/kbroulik/mountisorefactor into master

Since it's already inherently UDisks-specific, there's no real point in using the inherently synchronous Solid APIs. Instead, use asynchronous DBus calls all the way.

Also, fix the "unmount" action by deleting the loop device and then unmount it so it goes away from Places panel.

Also, allow mounting generic disk images, not just ISO images.

@kchoi @davidedmundson @bruns

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