QgsFields: Implement container methods

[qgib]

Author Name: Matthias Kuhn (@m-kuhn)
Original Redmine Issue: 8454

Redmine category:unknown

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Concerns C++ and Python.

C++: Implement the iterator and const_iterator things, so we can do e.g.:

Q_FOREACH( QgsField f, fields )

Python:
Should this be a dict or a list? Either

for fieldname, field in fields.iteritems():

or

for field in fields:

should be possible

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• easy_fix was configured as 0

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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• category_id was configured as Unknown