further touchup on kotlin conversion

[avalanchas]

• use more idiomatic language where possible, let-access, kotlin function calls, safe-access operators where possible
• fix endless loop during panelstate set
• method deprecation where possible, no breaking api changes for Java consumers
• make class "open", wasn't final before, shouldn't be now, even has protected methods
• remove companion object, unnecessary instance cost
• touchup javadoc where necessary
• fix several bugs caused by improper nullity access

demo app now also in working state @hannesa2 are there any outstanding issues to merge this into master? please let me know, happy to help if I can