Change TipPanel to be a shared, singleton instance.

Also make linux build use wxPopupWindow.

src/AudacityApp.cpp

@@ -66,6 +66,7 @@ It handles initialization and termination by subclassing wxApp.
 #include "effects/LoadEffects.h"
 #include "effects/Contrast.h"
 #include "effects/VST/VSTEffect.h"
+#include "widgets/ASlider.h"
 #include "FFmpeg.h"
 #include "GStreamerLoader.h"
 #include "Internat.h"
@@ -267,6 +268,8 @@ void QuitAudacity(bool bForce)
       }
    }
 
+   LWSlider::DeleteSharedTipPanel();
+
    ModuleManager::Dispatch(AppQuiting);
 
    wxLogWindow *lw = wxGetApp().mLogger;