[NestedListHelper] Fix indentation of selection, add tests

Summary:
When we try to increase / decrease an indentation level of a selection, just change
indentation level of each block in the selection, and then make sure all styles
are consistent via reformatList.

This actually fixes indentation of selection: previously all blocks had the same indentation after this action.

Add a unit test that covers as much weird selections & indentation cases as I could imagine.

Slightly overlaps with D29208: [NestedListHelper] Improve indentation code, so this one should be applied after that.

Test Plan: make && ctest

Reviewers: Frameworks, dfaure, mlaurent

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Differential Revision: https://phabricator.kde.org/D29210