Fix wxArrayString ctor, wstring concat, and wxList::Node for wx 3.3

- Plater: use Add() instead of wxArrayString(size_t, wxString) ctor
- Search: change sep from std::wstring to wxString for concatenation
- SendMultiMachinePage: replace wxList::Node* with compatibility_iterator
  (Node type removed in wx 3.3 with wxUSE_STD_CONTAINERS=ON)

src/slic3r/GUI/Search.cpp

@@ -184,7 +184,7 @@ bool OptionsSearcher::search(const std::string &search, bool force /* = false*/,
     found.clear();
 
     bool         full_list = search.empty();
-    std::wstring sep       = L" : ";
+    wxString sep       = L" : ";
 
     auto get_label = [this, &sep](const Option &opt, bool marked = true) {
         std::wstring out;