Improve arrow key navigation for right-to-left languages
While using right-to-left languages most of Dolphin is mirrored. However, the logic of what happens when the arrow keys are pressed to move between items in the main view was never adapted to account for that. Basically nothing works as expected because of this. It's more like dealing with a psychopath who misinterprets every command you give: Left is right, right is left, up is most of the time right but sometimes not, down is most the time left but sometimes not.
This commit fixes and adapts the logic if a right-to-left layout is used. This fully fixes arrow key navigation in icon view mode and compact view mode, though compact view mode still has more issues with right-to-left layout direction which aren't addressed here.
The test cases for keyboard navigation are adapted to also test the right-to-left layout direction under various conditions.
There is also a small change to make the test failure error output more human-readable.
This work for the benefit of the minority that use right-to-left languages both in Europe and the world is sponsored by NLnet and the European Commission which I think is beautfiul.
Note to reviewers
Disable "Show whitespace changes" in the Gitlab UI or this merge request will look way bigger than it is.