[Mount ISO Action] Optimize getDeviceFromBackingFile

Only query for StorageAccess as only this device type can be mounted, safes around 2/3 of needlessly created Devices significantly speeding up the lookup.

Also use property() instead of allProperties(), the latter of which does an explicit GetAll DBus query. Effectively, will probably not do much because it will be the first time the Device properties are queried but it's still slightly convoluted.

@bruns @kchoi

Right-clicking ISO files is still slow but a lot less so.

Mounting and unmounting (and determining whether it's mounted) an ISO file still works.

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