applet: hide current output device from tooltip when it's the only one
requested to merge ngraham/plasma-pa:hide-output-name-from-tooltip-when-theres-only-one-output into master
When there's only one output device, the user knows which device is playing audio, so there's no need to show it in the tooltip.
There are no changes to the tooltip text when there's more than one output device.
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