component validation enhancements

Currently YComponent#validate just returns void.

Validation can be improved in following ways:

• YComponent#validate returns boolean
  -> true : validation successfull, proceed as usual
  -> false: validation fails; don't render component
  or
• YComponent#validate returns void but method body may call YComponent#setValidationState(...) for an enhanced validation (error) result

• YComponent#validate throws an exception
  -> don'r render component; log exception (console) but don't show it in browser

Validation is very low level and only assures, that while rendering the component no exception is thrown/visible at browser.
For a more accurate validation handling each component has to implement it and each component view has to evaluate it.