Free ScrubbingToolBar from cycles...

... by having it update itself in idle time, not needing a push from Scrubber

Shrinking that remnant of the big strongly connected component to 6

src/toolbars/ScrubbingToolBar.cpp

@@ -52,6 +52,7 @@ EVT_COMMAND_RANGE( STBFirstButton,
                   STBFirstButton + STBNumButtons - 1,
                   wxEVT_COMMAND_BUTTON_CLICKED,
                   ScrubbingToolBar::OnButton )
+EVT_IDLE( ScrubbingToolBar::OnIdle )
 END_EVENT_TABLE()
 
 //Standard contructor
@@ -223,7 +224,6 @@ void ScrubbingToolBar::EnableDisableButtons()
    seekButton->SetEnabled(true);
 
    AudacityProject *p = &mProject;
-   if (!p) return;
 
    auto &scrubber = Scrubber::Get( *p );
    const auto canScrub = scrubber.CanScrub();
@@ -262,6 +262,12 @@ void ScrubbingToolBar::EnableDisableButtons()
    scrubber.CheckMenuItems();
 }
 
+void ScrubbingToolBar::OnIdle( wxIdleEvent &evt )
+{
+   evt.Skip();
+   EnableDisableButtons();
+}
+
 static RegisteredToolbarFactory factory{ ScrubbingBarID,
    []( AudacityProject &project ){
       return ToolBar::Holder{ safenew ScrubbingToolBar{ project } }; }

src/toolbars/ScrubbingToolBar.h

@@ -72,6 +72,8 @@ private:
    wxImage *downImage;
    wxImage *hiliteImage;
 
+   void OnIdle( wxIdleEvent &evt );
+
 public:
 
    DECLARE_CLASS(ScrubbingToolBar)