[Change] one callbackBridge with switch to j2m_EventName private fcns

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Add] UIContextMenu property

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>
[Minor] properties fiddling

Signed-off-by: Oleg Komarov <o.komarov11@imperial.ac.uk>