Improve code readability by requiring iteratable Group
to be named For.
The For's c'tor requires an iterator element as a first arg.
The For's c'tor allows for passing exactly one iterator element.
It's not possible to place iterators inside Group element anymore.
Change-Id: I9dfe2c0da058abac161f66c4e336da2417c383f1
Reviewed-by: hjk <hjk@qt.io>