Beat sync tempo slider UI

Current problem: sliding the tempo slider to fish around for the current BPM after a sync slave deck becomes the master deck is confusing and easy to overshoot

For high end controllers:

-Use a mechanically automated slider for tempo control that works the way motorized sliders do on recording studio mix boards. When you change the tempo on the master deck, the tempo slider on synchronized decks also physically moves to match. A dedicated tempo range button or knob also has lights that indicate the currently selected range.

For midrange controllers:

-Use a vertical jog wheel (the kind that runs perpendicular to your thumb) and a numeric LED or LCD to indicate current tempo. When the master tempo changes, the slave tempo also changes. A switch next to the wheel engages or disengages 1-BPM click stops on the wheel.

For current controllers:

-When a slave deck becomes a master deck, Rekordbox displays a large onscreen realtime "ghost" indicator of the current slider position, and the position required to match the currently playing BPM, with an arrow indicating the direction to move.