Bug 1508 - Scrubbing: Release mouse to exit Scrub Mode started by click & drag in Scrub Ruler 'no longer working'

It WAS working, just not the way QA wanted, because releasing mouse click when in the ruler and on the first seek continued the seek/scrub cycle.  Introduced a 'mInOneShotMode' variable to complete the baroque behaviour in the way that was requested.  Also updated tool tip code to reflect the one-shot and non one-shot difference in tooltip and status.

src/tracks/ui/Scrubbing.cpp

@@ -194,6 +194,8 @@ Scrubber::Scrubber(AudacityProject *project)
    , mProject(project)
    , mPoller { std::make_unique<ScrubPoller>(*this) }
    , mOptions {}
+   , mInOneShotMode( false )
+
 {
    if (wxTheApp)
       wxTheApp->Connect

src/tracks/ui/Scrubbing.h

@@ -138,6 +138,9 @@ public:
    void Pause(bool paused);
    bool IsPaused() const;
    void CheckMenuItems();
+   // Bug 1508
+   bool IsOneShotSeeking()const { return mInOneShotMode && IsScrubbing();};
+   bool mInOneShotMode;
 
 private:
    void DoScrub(bool seek);