Cover the filename display following the selected camera

[mattdawkins]

• The filename display above the timeline reads the selected camera's media controller, so selecting the other camera of a rig shows that camera's current filename. Adds a component test pinning that behaviour (it is easy to mistake for "always the left image" when both cameras share basenames, e.g. left/f0028.jpg and right/f0028.jpg).
• No behaviour change.

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