i3-like split navigation

[devgioele]

Organizing window splits like i3 (sway) does would be more user-friendly.
It consists in building a tree and remembering the last focused split in a subtree.
The following example demonstrates my issue:

Suppose 1 vertical and 2 horizontal splits have been made, resulting in the following logical tree:

               [ V ]
              /     \
         [ H ]       [ H ]
        /     \     /     \
   [ 1 ]   [ 3 ]   [ 2 ]   [ 4 ]

Graphically, the following 4 splits are seen:

-------------
|  x  |     |
|  1  |  2  |
-------------
|     |     |
|  3  |  4  |
-------------

and that the cursor is in the split marked with an x.

The cursor is then moved to the right split, resulting in:

-------------
|     |  x  |
|  1  |  2  |
-------------
|     |     |
|  3  |  4  |
-------------

The cursor is then moved to the split below, resulting in:

-------------
|     |     |
|  1  |  2  |
-------------
|     |  x  |
|  3  |  4  |
-------------

Moving the cursor back to the left split should result in the cursor being in the left top split:

-------------
|  x  |     |
|  1  |  2  |
-------------
|     |     |
|  3  |  4  |
-------------

What happens instead is that the cursor moves to the left bottom split:

-------------
|     |     |
|  1  |  2  |
-------------
|  x  |     |
|  3  |  4  |
-------------

The issue is related to how helix manages splits and their navigation. It is not a bug in the specific case shown. On a transposed tree of splits, the transposed problem exists.