Add tabs.close_mouse_button_on_bar ignore option

qutebrowser/config/configdata.yml

@@ -1135,6 +1135,7 @@ tabs.close_mouse_button_on_bar:
       - new-tab: "Open a new tab."
       - close-current: "Close the current tab."
       - close-last: "Close the last tab."
+      - ignore: "Don't do anything."
   desc: Behavior when the close mouse button is pressed on the tab bar.
 
 tabs.favicons.scale:

qutebrowser/mainwindow/tabwidget.py

@@ -438,7 +438,9 @@ class TabBar(QTabBar):
             idx = self.tabAt(e.pos())
             if idx == -1:
                 action = config.val.tabs.close_mouse_button_on_bar
-                if action == 'new-tab':
+                if action == 'ignore':
+                    return
+                elif action == 'new-tab':
                     self.new_tab_requested.emit()
                     return
                 elif action == 'close-current':