add_webapp: Reset description

Small little correction to the previous commit.

Author: clefebvre

usr/lib/webapp-manager/webapp-manager.py

@@ -333,6 +333,7 @@ def on_ok_button(self, widget):
 
     def on_add_button(self, widget):
         self.name_entry.set_text("")
+        self.desc_entry.set_text("")
         self.url_entry.set_text("")
         self.customparameters_entry.set_text("")
         self.icon_chooser.set_icon("webapp-manager")