move getOrCreateClient from sync.js to client.js

lib/client.js

@@ -2277,6 +2277,54 @@ MatrixClient.prototype.getFilter = function(userId, filterId, allowCached) {
     });
 };
 
+/**
+ * @param {string} filterName
+ * @param {Filter} filter
+ * @return {Promise<String>} Filter ID
+ */
+MatrixClient.prototype.getOrCreateFilter = function(filterName, filter) {
+
+    var filterId = this.store.getFilterIdByName(filterName);
+    var promise = q();
+    var self = this;
+
+    if (filterId) {
+        // check that the existing filter matches our expectations
+        promise = self.getFilter(self.credentials.userId,
+                         filterId, true
+        ).then(function(existingFilter) {
+            var oldDef = existingFilter.getDefinition();
+            var newDef = filter.getDefinition();
+
+            if (utils.deepCompare(oldDef, newDef)) {
+                // super, just use that.
+                // debuglog("Using existing filter ID %s: %s", filterId,
+                //          JSON.stringify(oldDef));
+                return q(filterId);
+            }
+            // debuglog("Existing filter ID %s: %s; new filter: %s",
+            //          filterId, JSON.stringify(oldDef), JSON.stringify(newDef));
+            return;
+        });
+    }
+
+    return promise.then(function(existingId) {
+        if (existingId) {
+            return existingId;
+        }
+
+        // create a new filter
+        return self.createFilter(filter.getDefinition()
+        ).then(function(createdFilter) {
+            // debuglog("Created new filter ID %s: %s", createdFilter.filterId,
+            //          JSON.stringify(createdFilter.getDefinition()));
+            self.store.setFilterIdByName(filterName, createdFilter.filterId);
+            return createdFilter.filterId;
+        });
+    });
+};
+
+
 /**
  * Gets a bearer token from the Home Server that the user can
  * present to a third party in order to prove their ownership