macosx: Fix media source items becoming unresponsive after switching away from media source views

Using NSImage's name property to store the MRL for a path control item's resepctive input item would cause asserts to fail when grabbing the input node from the relevant path control item.

Behind the scenes it seems the path control recreates the image without the right image name applied. However, this does not happen with accessibilityDescription -- so let's use that instead.

Signed-off-by: Claudio Cambra developer@claudiocambra.com