Development - xpdota/event-trigger GitHub Wiki
Most of the interesting stuff is in the xivsupport sub-module.
You can run GuiMain directly from your IDE for testing. This is the recommended way to run while developing.
There is an included run profile for IntelliJ:

Note that this run profile loads a different settings file (triggevent-testing.properties), so you don't have to worry about messing up your live settings.
To perform a full build, do mvn clean install at the root.