[Bug 647] Snap-To clicks find nearest snap point, not nearest previous.

Convert SnapTo from on/off to a choice of off/nearest/prior.

src/Snap.h

@@ -23,6 +23,13 @@
 
 class TrackClipArray;
 
+enum
+{
+   SNAP_OFF,
+   SNAP_NEAREST,
+   SNAP_PRIOR
+};
+
 class SnapPoint {
  public:
    SnapPoint(double t, Track *track) {
@@ -53,6 +60,11 @@ class SnapManager {
              bool *snappedPoint,
              bool *snappedTime);
 
+   static wxArrayString GetSnapLabels();
+   static wxArrayString GetSnapValues();
+   static const wxString & GetSnapValue(int index);
+   static int GetSnapIndex(const wxString & value);
+
  private:
    void CondListAdd(double t, Track *tr);
    double Get(int index);