Discard DocLeaf - use new pane_leaf() instead.

Discard DocLeaf - use new pane_leaf() instead.

pane_leaf() follows children of the pane as long as there is only one
with ->z==0.  Returns the final pane found.

Use this together with DocPane instead the hackish DocLeaf.

Possible some pane_focus() should be changed to pane_leaf().

Signed-off-by: NeilBrown <neil@brown.name>