Put Q_DECLARE_PRIVATE in the beginning of the class

This seems to be a usual practice by the majority of the Qt classes. There are
a few examples against it, but that is the minority.

Change-Id: I92f47f283a6cd279bc978761cb1ee375a514eeba
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>