Fix NotebookApp startup error on Windows

Convert all backslashes into forward slashes. This fixes the following startup exception, due to mixing forward and back slashes:
```
[E 10:34:29.657 NotebookApp] Exception while loading config file C:\Users\sftnight\.rootnb\jupyter_notebook_config.py
    Traceback (most recent call last):
      File "C:\Python38-32\lib\site-packages\traitlets\config\application.py", line 738, in _load_config_files
        config = loader.load_config()
      File "C:\Python38-32\lib\site-packages\traitlets\config\loader.py", line 614, in load_config
        self._read_file_as_dict()
      File "C:\Python38-32\lib\site-packages\traitlets\config\loader.py", line 646, in _read_file_as_dict
        exec(compile(f.read(), conf_filename, 'exec'), namespace, namespace)
      File "C:\Users\sftnight\.rootnb\jupyter_notebook_config.py", line 6
        c.NotebookApp.extra_static_paths = ['C:/Users/sftnight/build/release\js\']
```

Author: bellenot

main/src/nbmain.cxx

@@ -132,13 +132,16 @@ static bool CreateJupyterConfig(string dest, string rootbin, string rootlib, str
    ofstream out(jupyconfig, ios::trunc);
    if (out.is_open()) {
       out << "import os" << endl;
-      out << "rootbin = '" << rootbin << "'" << endl;
-      out << "rootlib = '" << rootlib << "'" << endl;
 #ifdef WIN32
-      string jsrootsys = rootdata + "\\js\\";
-      out << "os.environ['PYTHONPATH']      = '%s' % rootlib + ':' + os.getenv('PYTHONPATH', '')" << endl;
-      out << "os.environ['PATH']            = '%s:%s\\bin' % (rootbin,rootbin) + ':' + '%s' % rootlib + ':' + os.getenv('PATH', '')" << endl;
+      std::replace( rootbin.begin(), rootbin.end(), '\\', '/');
+      std::replace( rootdata.begin(), rootdata.end(), '\\', '/');
+      out << "rootbin = '" << rootbin << "'" << endl;
+      string jsrootsys = rootdata + "/js/";
+      out << "os.environ['PYTHONPATH']      = '%s' % rootbin + ';' + os.getenv('PYTHONPATH', '')" << endl;
+      out << "os.environ['PATH']            = '%s;%s/bin' % (rootbin,rootbin) + ';' + os.getenv('PATH', '')" << endl;
 #else
+      out << "rootbin = '" << rootbin << "'" << endl;
+      out << "rootlib = '" << rootlib << "'" << endl;
       string jsrootsys = rootdata + "/js/";
       out << "os.environ['PYTHONPATH']      = '%s' % rootlib + ':' + os.getenv('PYTHONPATH', '')" << endl;
       out << "os.environ['PATH']            = '%s:%s/bin' % (rootbin,rootbin) + ':' + os.getenv('PATH', '')" << endl;