GUI slows down as Output tab fills with messages

[danilovesky]

When the Output tab fills with a lot of messages (over 1,000 lines) Workcraft UI becomes unresponsive.
This is most likely due to the highlights associated with the messages, which are quite resource hungry.
A solution is to limit the length of Output log to the last N lines (e.g. 1,000 by default and configurable by the user).