api,modem: new 'SetPowerState()' method and 'PowerState' property

Going into/outof low-power state is now a user-requested action.

introspection/org.freedesktop.ModemManager1.Modem.xml

@@ -124,6 +124,18 @@
       <arg name="code" type="s" direction="in" />
     </method>
 
+    <!--
+        SetPowerState:
+	@state: A <link linkend="MMModemPowerState">MMModemPowerState</link> value, to specify the desired power state.
+
+        Set the power state of the modem. This action can only be run when the
+	modem is in <link linkend="MM-MODEM-STATE-DISABLED:CAPS"><constant>MM_MODEM_STATE_DISABLED</constant></link>
+	state.
+    -->
+    <method name="SetPowerState">
+      <arg name="state" type="u" direction="in" />
+    </method>
+
     <!--
         SetAllowedModes:
         @modes: Bitmask of <link linkend="MMModemMode">MMModemMode</link> values, to specify all the modes allowed in the modem.
@@ -377,6 +389,14 @@
     -->
     <property name="OwnNumbers" type="as" access="read" />
 
+    <!--
+        PowerState:
+
+	A <link linkend="MMModemPowerState">MMModemPowerState</link> value
+	specifying the current power state of the modem.
+    -->
+    <property name="PowerState" type="u" access="read" />
+
     <!--
         SupportedModes: