Zoom Toggle

Added preferences for Zoom-Toggle and put into menus.
New helper functions for determining zoom scaling.

Optional EXPERIMENTAL_ZOOM_TOGGLE_BUTTON added, and Light theme updated.
Not enabled for 2.2.2.

src/WaveTrack.h

@@ -577,10 +577,23 @@ class AUDACITY_DLL_API WaveTrack final : public PlayableTrack {
    // Only two types of sample display for now, but
    // others (eg sinc interpolation) may be added later.
    enum SampleDisplay {
-      LinarInterpolate = 0,
+      LinearInterpolate = 0,
       StemPlot
    };
 
+   // Various preset zooming levels.
+   enum ZoomPresets {
+      kZoomToFit = 0,
+      kZoomToSelection,
+      kZoomDefault,
+      kZoomMinutes,
+      kZoomSeconds,
+      kZoomMilliSeconds,
+      kZoomSamples,
+      kZoom4To1,
+      kMaxZoom,
+   };
+
    // Read appropriate value from preferences
    static WaveTrackDisplay FindDefaultViewMode();