Fixed size of header's right button

ubl-settings-bootloader.glade

@@ -1354,6 +1354,7 @@
                 <child>
                   <object class="GtkMenuButton" id="btnSettings">
                     <property name="visible">True</property>
+                    <property name="app-paintable">True</property>
                     <property name="can-focus">True</property>
                     <property name="focus-on-click">False</property>
                     <property name="receives-default">True</property>
@@ -1363,6 +1364,9 @@
                     <child>
                       <placeholder/>
                     </child>
+                    <style>
+                      <class name="noborder"/>
+                    </style>
                   </object>
                   <packing>
                     <property name="expand">False</property>