Fix issues of the "previous_engine" hotkey.

Check global_previous_engine_name when the previous engine is requested and rewrite it if necessary.
This change ensures that the "previous_engine" hotkey (Ctrl+space on Chrome OS) always works
whenever two or more engines are preloaded.

Currently, Ctrl+space could become NOP by the following scenarios (http://crosbug.com/6609):

(1)
1. preload engines A, B, C.
2. switch to A.
3. switch to B.
4. at this point, ctrl+space works fine. It selectes A and B alternately.
5. switch to A.
6. remove B from the preloaded engline list.
7. at this point, ctrl+space suddenly becomes NOP although C is still preloaded.
This is because the callback function of ctrl+space detects
global_previous_engine_name (i.e. B) is removed and automatically selects A (not
C) as a global_previous_engine_name.

(2)
1. preload engines A, B.
2. switch to A.
3. switch to B.
4. at this point, ctrl+space works fine. It selectes A and B alternately.
5. switch to A.
6. remove B from the preloaded engline list.
7. press ctrl+space several times, confirm that nothing happens (this is
expected since only one engine, A, is preloaded).
8. preload additional engine, C.
9. at this point, ctrl+space is still NOP although two engines, A and C, are
preloaded.

bus/ibusimpl.c

@@ -1083,6 +1083,7 @@ bus_ibus_impl_context_request_previous_engine (BusIBusImpl     *ibus,
                                                BusInputContext *context)
 {
     gchar *engine_name = NULL;
+
     if (!ibus->use_global_engine) {
         engine_name = (gchar *) g_object_get_data (G_OBJECT (context), "previous-engine-name");
     }
@@ -1091,6 +1092,16 @@ bus_ibus_impl_context_request_previous_engine (BusIBusImpl     *ibus,
             ibus->global_previous_engine_name = bus_ibus_impl_load_global_previous_engine_name_from_config (ibus);
         }
         engine_name = ibus->global_previous_engine_name;
+        if (engine_name != NULL) {
+            /* If the previous engine is removed from the engine list or the
+               current engine and the previous engine are the same one, force
+               to pick a new one. */
+            if (!_find_engine_desc_by_name (ibus, engine_name) ||
+                g_strcmp0 (engine_name, ibus->global_engine_name) == 0) {
+                g_free (engine_name);
+                ibus->global_previous_engine_name = engine_name = NULL;
+            }
+        }
     }
 
     /*