pavucontrol's "on" text string should be aligned/close to the device output button, or replaced by something clearer

The "on" string's positioning looks awkward in many cases:

Either it could be split into its own string/cell and right-aligned, or maybe the button's label could include the string (in two variants: "on %output_device_name" and "from %input_device_name"), or maybe we could do away entirely with the "on" label and let the button have an icon that represents either a microphone, headphone, or speaker to the left of its label. Another alternative is for the "on" label to be replaced by some unicode arrows, → for output to that device or ← for input from that device.