Implement checkbox state management methods

[XxXAdvisaryXxX]

Added methods to manage checkbox state.

[Torwent]

This is kind of redundant after I added the Checked property to the checkboxes:

{$INCLUDE WaspLib/main.simba}

var
  check: TLazCheckBox;
begin
-  check.SetChecked(True);
-  WriteLn check.IsChecked();
-  check.Toggle();
+  check.Checked := True;
+  WriteLn check.Checked;
+  check.Checked := not check.Checked;
end.    

They are even shorter than the functions with the exception of toggle