This patch implements an iterator that returns a pair containing both the
key and the value of an entry in QHash/QMap.
[ChangeLog][QtCore][Containers] Added an stl-like iterator to go through
QHash/QMap returning both the key and the value of the element pointed to.
That lets QHash/QMap interoperate better with stl's algorithms like
std::set_union.
Change-Id: Idbf8a8581510b3493648c34ab04c556de9fa4aa7
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>