Closing tab on middle mouse button press

Motivation: Most of the applications with tabs support such way
of closing tabs.

terminatorlib/notebook.py

@@ -568,6 +568,8 @@ class TabLabel(Gtk.HBox):
         self.terminator = Terminator()
         self.config = Config()
 
+        self.connect("button-press-event", self.on_button_pressed)
+
         self.label = EditableLabel(title)
         self.update_angle()
 
@@ -650,4 +652,8 @@ class TabLabel(Gtk.HBox):
         """The close button has been clicked. Destroy the tab"""
         self.emit('close-clicked', self)
 
+    def on_button_pressed(self, _widget, event):
+        if event.button == 2:
+            self.on_close(_widget)
+
 # vim: set expandtab ts=4 sw=4: