api: new SetAllowedBands() to be able to modify the allowed bands in the modem

Changing the allowed bands in a modem may fail, for example if trying to set a
frequency band which is not in the bands mask reported as Supported by the modem.
Therefore, we need an explicit SetAllowedBands() method with proper error
reporting instead of making the property writable.

new/org.freedesktop.ModemManager1.Modem.xml

@@ -118,6 +118,20 @@
       </arg>
     </method>
 
+    <method name="SetAllowedBands">
+      <tp:docstring>
+        Set the radio frequency and technology bands the device is currently
+        allowed to use when connecting to a network.
+      </tp:docstring>
+      <annotation name="org.freedesktop.DBus.GLib.Async" value=""/>
+      <annotation name="org.freedesktop.DBus.GLib.CSymbol" value="impl_modem_set_allowed_band"/>
+      <arg name="mode" type="u" tp:type="MM_MODEM_BAND">
+        <tp:docstring>
+          Bitmask of all the bands allowed in the modem.
+        </tp:docstring>
+      </arg>
+    </method>
+
     <!-- SIGNALS -->
 
     <signal name="StateChanged">
@@ -289,11 +303,11 @@
       </tp:docstring>
     </property>
 
-    <property name="AllowedBands" type="t" access="readwrite" tp:type="MM_MODEM_BAND">
+    <property name="AllowedBands" type="t" access="read" tp:type="MM_MODEM_BAND">
       <tp:docstring>
-        The allowed radio frequency and technology bands the device is currently
-        allowed to use when connecting to a network.  For POTS devices, only
-        the "any" value is supported.
+        The radio frequency and technology bands the device is currently allowed
+        to use when connecting to a network.  For POTS devices, only the "any"
+        value is supported.
       </tp:docstring>
     </property>