EVENT_DEFAULT type for party messages, new information message for leader

Party messages are displayed with the EVENT_DEFAULT type. There is an information message in Tibia for a leader, it's display situation seems to be kinda random, so I have introduced it like this.
This line will be easier to understand as:
```
<< "In this party, shared experience is currently " << (sharedExpActive ? "active" : "inactive") << ".";
```
There's another instance of this on line 243 too.