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Use flake8 to enforce code style

Flake8 requires at least 3.4

Use flake8 to make the code mostly pep8 compliant. Only exception is line
length, which is set to 120.

Handling for attributes and nested components

Remove export of non-component children of components.

Introduce QmlBaseComponent and make QmlClass and a new class, QmlComponent,
inherit from it.

Split QmlClass.__str__() to avoid the multiple `if`s.

This is Python, not C++ :)

No need to return the created object, it's not used.

Increase coverage

Inline comment fixes