Allow '.' characters in event field names to be escaped with '\' so that

fields including a '.' can be included in a filter.

I considered replacing '.' with '/'. Since '/' was less likely to appear
in event field names. However if we used '\' to escape a literal '/' we
risk confusing it with the JSON escape '\/'.

api/client-server/v2_alpha/definitions/sync_filter.json

@@ -39,7 +39,7 @@
     "event_fields": {
       "type": "array",
       "description":
-        "List of event fields to include. If this list is absent then all fields are included. The entries may include '.' charaters to indicate sub-fields. So ['content.body'] will include the 'body' field of the 'content' object. A server may include more fields than were requested.",
+        "List of event fields to include. If this list is absent then all fields are included. The entries may include '.' charaters to indicate sub-fields. So ['content.body'] will include the 'body' field of the 'content' object. A literal '.' character in a field name may be escaped using a '\\'. A server may include more fields than were requested.",
       "items": {
         "type": "string"
       }