Fix narrow/wide string concatenation errors for wxWidgets 3.3

In wx 3.3 with wxUSE_STD_CONTAINERS=ON, wxString is backed by
std::wstring, so direct concatenation of const char[] with
std::wstring or wxUniCharRef fails. Fix by splitting compound
concatenations into separate += operations on wxString, or by
wrapping the left operand in wxString() to use its operator+.

Files fixed:
- AuxiliaryDataViewModel.cpp: split "\\" + wxString/wstring chains
- AboutDialog.cpp: split std::string("\n") + wxUniCharRef
- Auxiliary.cpp: wrap dir.wstring() in wxString(), split "/" + wstring

src/slic3r/GUI/AboutDialog.cpp

@@ -293,7 +293,8 @@ AboutDialog::AboutDialog()
                     find_txt += text_list[i][o];
                     count_txt += text_list[i][o];
                 } else {
-                    find_txt += std::string("\n") + text_list[i][o];
+                    find_txt += "\n";
+                    find_txt += text_list[i][o];
                     count_txt = text_list[i][o];
                 }
             }