Custom arrow keys handling

[szymonkups]

Extracted from: https://github.com/ckeditor/ckeditor5-engine/issues/696#issuecomment-263840273.

Custom widget handler for arrows

Use the view events (keydown or selectionChange).

Object selected

1. Collapse selection in the right direction (or even base on the view#selectionChange which currently implements the same behaviour).
2. Use Document#getNearestSelectionPosition with the right direction param to find the next possible position.
3. If not position found... rollback?

Selection next to an object

1. If selection is not collapsed – do nothing.
2. Two options:

• Use DataController#extendSelection() on a selection clone to probe what's next. If selection focus is before/after an object, it means that this object should be selected.
• Alternatively – use the walker to find a closest object (in the right direction, ofc). Check if position after/before that object is touching the current selection.

3. Select the found object.

Note: The above algorithm (the option 2a) will automatically work with the case when e.g. we selected an object at the beginning of the root and pressed left arrow. The focus will not be moved any further, so on its right side there'll still be that object and we'll reselect it. 🎉