Fix renaming of notebook tab label during collaboration

When Pippy is shared and joined, the notebook tab label is changed from
"New source file" to "New_source_file.py" on the joined instance.  A
side-effect of the _purify_file method.

* send the notebook tab label to the peers without purifying,

* make the _purify_file method public as purify_name,

* when the "go" button is pressed, purify the file name from the label,

notebook.py

@@ -300,7 +300,7 @@ def _key_press_cb(self, widget, event):
     def set_current_label(self, label):
         child = self.get_nth_page(self.get_current_page())
         widget = self.get_tab_label(child)
-        widget.set_text(self._purify_file(label))
+        widget.set_text(self.purify_name(label))
 
     def set_current_path(self, path):
         child = self.get_nth_page(self.get_current_page())
@@ -317,7 +317,7 @@ def get_text_view(self):
         text_view = tab[0]
         return text_view
 
-    def _purify_file(self, label):
+    def purify_name(self, label):
         if not label.endswith('.py'):
             label = label + '.py'
 
@@ -334,7 +334,7 @@ def get_all_data(self):
         for i in range(0, self.get_n_pages()):
             child = self.get_nth_page(i)
 
-            label = self._purify_file(self.get_tab_label(child).get_text())
+            label = self.get_tab_label(child).get_text()
             names.append(label)
 
             path = self.get_tab_label(child).get_path()
@@ -354,7 +354,7 @@ def get_all_data(self):
     def get_current_file_name(self):
         child = self.get_nth_page(self.get_current_page())
         label = self.get_tab_label(child).get_text()
-        label = self._purify_file(label)
+        label = self.purify_name(label)
 
         return label
 

pippy_app.py

@@ -619,6 +619,7 @@ def _write_all_buffers(self, tmp_dir):
         data = self._source_tabs.get_all_data()
         zipdata = list(zip(data[0], data[1]))
         for name, content in zipdata:
+            name = self._source_tabs.purify_name(name)
             with open(os.path.join(tmp_dir, name), 'w') as f:
                 # Write utf-8 coding prefix if there's not one already
                 if re.match(r'coding[:=]\s*([-\w.]+)',