Separate rate limiter for Twitter blogs

[thomas694]

As mentioned in twitter blogs not limiting threads #454

Currently the rate limiter for Tumblr API is used for Twitter blogs too.
The connection settings dialog needs to be extended by a new row for Twitter rate limits.
And the TwitterCrawler needs to be adapted to use a new RateLimiter instance instead of the one created for Tumblr API.

[moxin3]

Error: You Should lower the connections to Tumblr API in the settings for twitter blogs