Use standard library style members of wxArrayString (and wxString) ...

... which will make it easier to change the types of those containers to
std::vectors of other string-like classes

for wxString,

IsEmpty => empty
Clear => clear
Alloc => reserve

for wxArrayString,

Count => size
GetCount => size
IsEmpty => empty
Add => push_back
Clear => clear
Empty => clear
Sort => std::sort (only with default comparator)
SetCount => resize
Last => back
Item => operator []
Alloc => reserve

src/UndoManager.cpp

@@ -384,7 +384,7 @@ void UndoManager::StateSaved()
 // currently unused
 //void UndoManager::Debug()
 //{
-//   for (unsigned int i = 0; i < stack.Count(); i++) {
+//   for (unsigned int i = 0; i < stack.size(); i++) {
 //      for (auto t : stack[i]->tracks->Any())
 //         wxPrintf(wxT("*%d* %s %f\n"),
 //                  i, (i == (unsigned int)current) ? wxT("-->") : wxT("   "),