api,dbus: new `IpTimeout' property in the Bearer interface

This IpTimeout property will be read by NetworkManager, and used as the time to
wait for pppd to establish the IP configuration (20s by default).

new/org.freedesktop.ModemManager1.Bearer.xml

@@ -219,6 +219,13 @@
     -->
     <property name="Ip6Config" type="a{sv}" access="read" />
 
+    <!--
+        IpTimeout:
+
+        Maximum time to wait for a successful IP establishment, when PPP is used.
+    -->
+    <property name="IpTimeout" type="u" access="read" />
+
     <!--
         Properties:
 

src/mm-bearer.c

@@ -34,6 +34,9 @@
 #include "mm-log.h"
 #include "mm-modem-helpers.h"
 
+/* We require up to 20s to get a proper IP when using PPP */
+#define MM_BEARER_IP_TIMEOUT_DEFAULT 20
+
 G_DEFINE_TYPE (MMBearer, mm_bearer, MM_GDBUS_TYPE_BEARER_SKELETON);
 
 enum {
@@ -650,6 +653,7 @@ mm_bearer_init (MMBearer *self)
     mm_gdbus_bearer_set_connected (MM_GDBUS_BEARER (self), FALSE);
     mm_gdbus_bearer_set_suspended (MM_GDBUS_BEARER (self), FALSE);
     mm_gdbus_bearer_set_properties (MM_GDBUS_BEARER (self), NULL);
+    mm_gdbus_bearer_set_ip_timeout (MM_GDBUS_BEARER (self), MM_BEARER_IP_TIMEOUT_DEFAULT);
     mm_gdbus_bearer_set_ip4_config (MM_GDBUS_BEARER (self),
                                     mm_common_bearer_ip_config_get_dictionary (NULL));
     mm_gdbus_bearer_set_ip6_config (MM_GDBUS_BEARER (self),