Fix AddBehavior being ignored when splitting multiple times in the same direction
Previously, when eg. hitting Ctrl+Shift+(
multiple times, the resulting
panes would look like:
-------------------------------------
| | | | |
| Pane 1 | Pane 4 | Pane 3 | Pane 2 |
| | | | |
-------------------------------------
This was caused by AddBehavior
being ignored when splitting more than
once in the same orientation. The new pane would always be created at
the index of the current pane. This commit fixes the problem so that
AddBehavior::AddAfter
will open a new pane downwards or to the right in
all scenarios.