Implement drag-scrub, compatibly with the existing move-scrub...

... Start scrub by click or double click on the scrub head; release button or
not; then move.

If you release before moving, you get scrubbing as before, controlled by
motion.  Click or drag to switch in and out of seeking.  Stop with ESC,
spacebar, etc.  No change of selection.

But now if you drag, then scrubbing contines until you release the mouse or
otherwise stop with a key.

If by release of the mouse, then the selection changes as if by a click at
the last play position.  If you hold shift, then, as if by shift-click.

If drag begins with a double-click, then the play head remains centered and
the track moves.

src/AudioIO.h

@@ -208,6 +208,10 @@ class AUDACITY_DLL_API AudioIO final {
    * Return true if some work was really enqueued.
    */
    bool EnqueueScrubBySignedSpeed(double speed, double maxSpeed, bool maySkip);
+
+   /** \brief return the ending time of the last enqueued scrub interval.
+   */
+   double GetLastTimeInScrubQueue() const;
 #endif
 
    /** \brief  Returns true if audio i/o is busy starting, stopping, playing,