Avoid using connect in QQuickImplicitSizeItem.

In the past connect has shown up as a noticeable cost in item
creation. Avoid the use here by refactoring to call base class
implementation and emit override signal as well.

Task-number: QTBUG-32764
Change-Id: If3e5a1cc55c113d812705fcfdc7a17aee9d6f51c
Reviewed-by: Alan Alpert (Personal) <416365416c@gmail.com>