logind: also expose bool prop on bus that declares whether we are on external power

The three core variables that affect idleness handling are whether we
are docked, whether we are on AC power and whether the lid is closed,
hence let's also expose the third variable on the bus, to make things
nicely debuggable.

src/login/logind-dbus.c

@@ -275,6 +275,7 @@ static int property_get_scheduled_shutdown(
 static BUS_DEFINE_PROPERTY_GET_ENUM(property_get_handle_action, handle_action, HandleAction);
 static BUS_DEFINE_PROPERTY_GET(property_get_docked, "b", Manager, manager_is_docked_or_external_displays);
 static BUS_DEFINE_PROPERTY_GET(property_get_lid_closed, "b", Manager, manager_is_lid_closed);
+static BUS_DEFINE_PROPERTY_GET_GLOBAL(property_get_on_external_power, "b", manager_is_on_external_power);
 static BUS_DEFINE_PROPERTY_GET_GLOBAL(property_get_compat_user_tasks_max, "t", CGROUP_LIMIT_MAX);
 static BUS_DEFINE_PROPERTY_GET_REF(property_get_hashmap_size, "t", Hashmap *, (uint64_t) hashmap_size);
 
@@ -2668,6 +2669,7 @@ const sd_bus_vtable manager_vtable[] = {
         SD_BUS_PROPERTY("ScheduledShutdown", "(st)", property_get_scheduled_shutdown, 0, 0),
         SD_BUS_PROPERTY("Docked", "b", property_get_docked, 0, 0),
         SD_BUS_PROPERTY("LidClosed", "b", property_get_lid_closed, 0, 0),
+        SD_BUS_PROPERTY("OnExternalPower", "b", property_get_on_external_power, 0, 0),
         SD_BUS_PROPERTY("RemoveIPC", "b", bus_property_get_bool, offsetof(Manager, remove_ipc), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("RuntimeDirectorySize", "t", NULL, offsetof(Manager, runtime_dir_size), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("InhibitorsMax", "t", NULL, offsetof(Manager, inhibitors_max), SD_BUS_VTABLE_PROPERTY_CONST),