how to use all the features of FEBuilder?
There is a wiki for manual.
(Recently, it seems there was a problem with the wiki server, it takes time to read.)
how to add new classes or how to insert animations some other stuff
Please do not extend the class.
Extension of class raises various problems.
Therefore, it is disabled by default.
Please do not extend, please reuse class data not to use.
Adding a new stuff is also a difficult remodeling.
I also recommend that you do not do it.