Bug 510 - undo can change which tracks are selected, and focus

Must more extensive review of saving state

Try this one David.  I reviewed all actions in Menus.cpp and have added state
saving where it seemed to be missing.  I had no idea it would be so many (22). 
And that was only reviewing Menus.cpp.  I believe that will get them all since
all keyboard actions are tied to a menu or command action and they all go
through Menus.cpp. (I still have a few to review in TrackPanel.cpp though)

src/UndoManager.cpp

@@ -186,6 +186,10 @@ bool UndoManager::RedoAvailable()
 
 void UndoManager::ModifyState(TrackList * l, double sel0, double sel1)
 {
+   if (current == wxNOT_FOUND) {
+      return;
+   }
+
    SonifyBeginModifyState();
    // Delete current
    stack[current]->tracks->Clear(true);