Re-implement "Customize entry types" dialog

In https://github.com/JabRef/jabref/pull/4716, the Customize entry types dialog was temporarily removed. It should be rei-mplemented in JavaFX once we converged to a stable entry editor layout (refs https://github.com/JabRef/jabref/issues/2790 https://github.com/JabRef/jabref/issues/366 https://github.com/JabRef/jabref/issues/4687 https://github.com/JabRef/jabref/issues/4688).