Changed lifetime management of UIHandle objects, no singletons...

... Rather, construct them during hit tests (also capturing more state sooner
rather than at Click time, and adding some accessors for later use)

This also fixes bug 1677 by other means and avoids similar problems.

A cell may be implemented to re-use a previously hit handle object, not yet
clicked, in a later hit test, by remembering a weak pointer, but TrackPanel
holds the strong pointers that determine when the object is destroyed.

And the objects will surely be destroyed after drag-release, or ESC key.

For now they are also destroyed whenever not dragging, and hit-testing is
re-invoked; that will be changed later, so that the re-use mentioned above
becomes effective, but still they will be destroyed when the pointer moves
from one cell to another.

src/TrackPanel.cpp

@@ -770,7 +770,7 @@ void TrackPanel::CancelDragging()
          ProcessUIHandleResult(
             this, mRuler, pTrack.get(), NULL, refreshResult);
       mpClickedTrack.reset();
-      mUIHandle = NULL;
+      mUIHandle.reset();
       Uncapture();
    }
 }
@@ -1465,7 +1465,7 @@ try
             (this, mRuler, pClickedTrack.get(), pTrack.get(), refreshResult);
          if (refreshResult & RefreshCode::Cancelled) {
             // Drag decided to abort itself
-            mUIHandle = NULL;
+            mUIHandle.reset();
             mpClickedTrack.reset();
             Uncapture( &event );
          }
@@ -1484,7 +1484,7 @@ try
          // Null this pointer out first before calling Release -- because on Windows, we can
          // come back recursively to this place during handling of the context menu,
          // because of a capture lost event.
-         mUIHandle = nullptr;
+         mUIHandle.reset();
          UIHandle::Result refreshResult =
             uiHandle->Release( tpmEvent, GetProject(), this );
          ProcessUIHandleResult
@@ -1553,7 +1553,7 @@ void TrackPanel::HandleClick( const TrackPanelMouseEvent &tpmEvent )
       UIHandle::Result refreshResult =
          mUIHandle->Click( tpmEvent, GetProject() );
       if (refreshResult & RefreshCode::Cancelled)
-         mUIHandle = NULL;
+         mUIHandle.reset();
       else
          mpClickedTrack = pTrack;
       ProcessUIHandleResult
@@ -3113,7 +3113,7 @@ TrackPanelCellIterator::TrackPanelCellIterator(TrackPanel *trackPanel, bool begi
          mpCell = mpTrack;
       else
          mpCell = trackPanel->GetBackgroundCell();
-      }
+   }
 
    const auto size = mPanel->GetSize();
    mRect = { 0, 0, size.x, size.y };