2.4.0

• Dependencies upgrade: Kotlin 1.3.60, serialization 0.14.0, Ktor 1.2.6, Spring Boot 2.2.1 and others
• Add new onEvent extension function and deprecate setEventListener method
• New addCssStyle, removeCssStyle, addSurroundingCssStyle and removeSurroundingCssStyle methods for use with Style objects
• Add focus() and blur() methods to the Button class
• Add centered property to all modals
• Support column groups in the Tabulator component (#111)