Simulation button should be checkable

Without this, when you go to airframe config and the current config is
Simulation it will not be selected. Hence you won’t know what the
current config is.

src/ui/px4_configuration/QGCPX4AirframeConfig.ui

@@ -358,6 +358,9 @@
            <property name="text">
             <string>Simulation Setup</string>
            </property>
+           <property name="checkable">
+            <bool>true</bool>
+           </property>
           </widget>
          </item>
          <item>