Introduced a class GOControl as a universal base for other controls

[oleg68]

This is a second PR related to #1816

It

• introduces a new class GOControl as a universal base for other controls
• changed the parameter type of ControlChanged from void * to GOControl *

It added a capability of making a dynamic_cast from GOControl to other classes in the future.

It is just refactoring. No GO behavior should be changed.

[oleg68]

@rousseldenis could you approve this PR?