core: allow to set a direction for a horizontal/vertical split

[aruhier]

Currently, a ViewNode can take a prefered_split_type which changes the behavior of the split when upgraded into a SplitNode (horizontal, vertical, tabbed, etc.).

However it should be possible to indicate a direction for horizontal and vertical splits: left/right or top/bottom of the node.

Idea of implementation

A new field in the ViewNode indicating if the next node is to be added before or after the current node in the children list could solve this issue. Before/After for a horizontal split would mean top/down, and left/right for a horizontal split.