Issue description

When you have a layout such as the following:

HSplit(
    VSplit(
        HSplit(Pane, Pane),
        HSplit(Pane, Pane)
    ),
    VSplit(
        HSplit(Pane, Pane),
        HSplit(Pane, Pane)
    )
)

(i.e, two 2x2 grids contained in a horizontal split)

From what I can gather there is no way to resize the center horizontal split outside of using the mouse, since resize-pane looks like it only resizes the splits in the containing cell and nothing further up the tree.

It would be great if there was a way to do this without using the mouse so that control mode clients can cope with this kind of resize.

(Even if it was just exposing layout_resize_layout, and having a way to do multiple resizes at once before the layout is reconstructed).