Add user interface for scrubbing and Experimental.h switches

EXPERIMENTAL_SCRUBBING_BASIC enables middle-click-drag to scrub, or to seek
when shift is down.

EXPERIMENTAL_SCRUBBING_SMOOTH_SCROLL adds middle-double-click-drag to
scrub or (with SHIFT down) to seek while keeping the playhead at the midline
of the track display.

EXPERIMENTAL_SCRUBBING_SCROLL_WHEEL allows control wheel to vary the speed
limit, while scrubbing (not seeking).

All three are enabled in this commit.

Also notice the changes to status bar messages and the Mouse Preferences dialog.

Scrubbing works only in Select and Multi tools.

src/toolbars/ToolsToolBar.cpp

@@ -80,7 +80,12 @@ ToolsToolBar::ToolsToolBar()
    wxASSERT( drawTool     == drawTool     - firstTool );
    wxASSERT( multiTool    == multiTool    - firstTool );
 
+#ifdef EXPERIMENTAL_SCRUBBING_BASIC
+   mMessageOfTool[selectTool] =
+      _("Click and drag to select audio, Middle-Click and drag to scrub, Shift-Middle-Click and drag to seek");
+#else
    mMessageOfTool[selectTool] = _("Click and drag to select audio");
+#endif
    mMessageOfTool[envelopeTool] = _("Click and drag to edit the amplitude envelope");
    mMessageOfTool[drawTool] = _("Click and drag to edit the samples");
 #if defined( __WXMAC__ )

src/toolbars/TranscriptionToolBar.h

@@ -99,6 +99,8 @@ class TranscriptionToolBar:public ToolBar {
    void SetEnabled(bool enabled);
    void SetPlaying(bool down, bool looped, bool cutPreview);
 
+   double GetPlaySpeed() const { return mPlaySpeed / 100.0; }
+
  private:
 
    void InitializeTranscriptionToolBar();