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/AuxiliaryDataViewModel.cpp

@@ -332,9 +332,12 @@ wxDataViewItemArray AuxiliaryModel::ImportFile(AuxiliaryModelNode* sel, wxArrayS
         // Copy imported file to project temp directory
         fs::path src_bfs_path(file_path.ToStdWstring());
         wxString dir_path = m_root_dir;
-        if (sel != m_root)
-            dir_path += "\\" + sel->name;
-        dir_path += "\\" + src_bfs_path.filename().generic_wstring();
+        if (sel != m_root) {
+            dir_path += "\\";
+            dir_path += sel->name;
+        }
+        dir_path += "\\";
+        dir_path += src_bfs_path.filename().generic_wstring();
 
         boost::system::error_code ec;
         if (!fs::copy_file(src_bfs_path, fs::path(dir_path.ToStdWstring()), fs::copy_options::overwrite_existing, ec))