Add code from merging.

This brings more of the code from DarkAudacity into Audacity, though not yet enabled.  This will make cherry picking later easier.
- Changing colour of html displays now possible, as colour links are visible for substitution.  (Can't use css as wxHTML is very limited).
- Export can now be preset to a particular format independent of preferences.
- SnapTo reversion on zoom now a DA controlled option.
- Caching of pinning preference for faster repaint because preferences are slow.
- Record Append now has sensible limit on number of tracks, rather than giving an error, and flipping the meaning of shift is now DA controlled.

src/ViewInfo.cpp

@@ -89,6 +89,18 @@ bool ZoomInfo::ZoomOutAvailable() const
 void ZoomInfo::SetZoom(double pixelsPerSecond)
 {
    zoom = std::max(gMinZoom, std::min(gMaxZoom, pixelsPerSecond));
+#ifdef EXPERIMENTAL_DA
+   // Disable snapping if user zooms in a long way.
+   // Helps stop users be trapped in snap-to.
+   // The level chosen is in sample viewing range with samples
+   // still quite close together.
+   if( zoom > (gMaxZoom * 0.06  ))
+   {
+      AudacityProject * project = GetActiveProject();
+      if( project )
+         project->OnSnapToOff();
+   }
+#endif
 }
 
 void ZoomInfo::ZoomBy(double multiplier)