callback - Added workaround for IAR issue with type information

In the IAR ide, implicitly generated structures based on function
templates end up with missing type information. This has no effect
on using the IAR compiler standalone, but when used through the ide
the missing type information causes the ide to error.

As a workaround, moved the function attributes generated for the
Callback and Event classes into the class scope. This avoids the
syntax that confuses IAR.

Author: geky