Rewrite undo handling for Stretch to be more like other drags...

... that is, push a state only at button-up.

The undo during each drag increment didn't actually happen, because of a check
in OnUndo whether there is a drag in progress!

But this is not necessary to fix the excessive number of undo items, because
commit 63ae687baf already did that by other means.

It only removes an unusual usage.

src/TrackPanel.h

@@ -327,7 +327,6 @@ class AUDACITY_DLL_API TrackPanel final : public OverlayPanel {
    struct StretchState {
       StretchEnum mMode { stretchCenter }; // remembers what to drag
       bool mStretching {}; // true between mouse down and mouse up
-      bool mStretched {}; // true after drag has pushed state
       double mStart {}; // time of initial mouse position, quantized
       // to the nearest beat
       double mSel0 {};  // initial sel0 (left) quantized to nearest beat