Defined EXPERIMENTAL_SCROLLING LIMITS

When it is enabled, the project can scroll up to one-half of a screenful
beyond time zero or the maximum track time.  I was careful to disable selection
of negative times.

This is motivated by the smooth scrolling scrub.  It behaves more sensibly at
the extremes.  It can still keep the play indicator centered.

Also removed an unused member of ViewInfo.

src/ViewInfo.h

@@ -33,7 +33,6 @@ struct ViewInfo {
    double screen;               // screen width in secs
    double total;                // total width in secs
    double zoom;                 // pixels per second
-   double lastZoom;
 
    // Current horizontal scroll bar positions, in pixels
    wxInt64 sbarH;
@@ -43,6 +42,7 @@ struct ViewInfo {
    // Internal wxScrollbar positions are only int in range, so multiply
    // the above values with the following member to get the actual
    // scroll bar positions as reported by the horizontal wxScrollbar's members
+   // i.e. units are scroll increments per pixel
    double sbarScale;
 
    // Vertical scroll step