Use underline for browse_for_client_cert_button

data/gui/accounts_window.ui

@@ -443,6 +443,7 @@
                                             <property name="visible">True</property>
                                             <property name="can_focus">True</property>
                                             <property name="receives_default">True</property>
+                                            <property name="use_underline">True</property>
                                             <signal name="clicked" handler="on_browse_for_client_cert_button_clicked"/>
                                           </object>
                                           <packing>