Fix/layout model

[martingrossmann]

Description

The additional information (images for actual, diff, expected) of the result of a pixel based layout check were only stored as a Map and were exported to the report in a JSON format.
This change introduces a LayoutCheckContext model which can be better handed by ReportNG and other platforms working with report data model.

Every LayoutCheckContext is linked to it's own ErrorContext. This allows as follows:

• more then one layout check assertion in a testcase.
• Mix of UiElement assertions and layout check assertion as collected assertions in one test case

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules