context initialization callback method

Especially usefull for YSessionContext.
Constructor can't be used as YFaces#getRequestContext() would create a cyclic reference

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1) Command Pattern
Constructor creates an 'InitializeCommand' and adds it to a command queue

Impl:
static List YRequestContextImpl#getCommandQueue()

con:

• more complex than delegation
• no real place where the command queue is managed

pro:

• no isInitialized/setInitialized member needed
• context is not applicable for calling any parents context#init()
• better options for future enhancements (e.g. init on pages, conversation etc)

2) Delegatation of init() from YRequest up to YApplication

Impl:
starting with YRequestContext#init() each context is responsible for calling the parents
context #init() but only when parent context wasn't already initialized

disadvantage:

• a custom override of #init() always have to call super() (error prone)
• no chance of 'jump over' a context instance (stop init always stops whole init process)
• at least one useless boolean check is done with every request

3) Right now olny session gets considered

pro:

• easy to implement
• 80% of all use-cases

con:

• missing flexibility for future enhancements