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/prefs/TracksPrefs.cpp

@@ -30,6 +30,7 @@
 
 #include "../Experimental.h"
 
+int TracksPrefs::iPreferencePinned = -1;
 
 namespace {
    const wxChar *PinnedHeadPreferenceKey()
@@ -179,11 +180,18 @@ void TracksPrefs::PopulateOrExchange(ShuttleGui & S)
 
 bool TracksPrefs::GetPinnedHeadPreference()
 {
-   return gPrefs->ReadBool(PinnedHeadPreferenceKey(), PinnedHeadPreferenceDefault());
+   // JKC: Cache this setting as it is read many times during drawing, and otherwise causes screen flicker.
+   // Correct solution would be to re-write wxFileConfig to be efficient.
+   if( iPreferencePinned >= 0 )
+      return iPreferencePinned == 1;
+   bool bResult = gPrefs->ReadBool(PinnedHeadPreferenceKey(), PinnedHeadPreferenceDefault());
+   iPreferencePinned = bResult ? 1: 0;
+   return bResult;
 }
 
 void TracksPrefs::SetPinnedHeadPreference(bool value, bool flush)
 {
+   iPreferencePinned = value ? 1 :0;
    gPrefs->Write(PinnedHeadPreferenceKey(), value);
    if(flush)
       gPrefs->Flush();