chat: remove unused methods from ChatEngineKadu

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
chat: ensure that AdiumStyle does not have converting contructors

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
chat: extract sublacces from chat engines

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>