`Tree::simplify_children_of_tile` does not respect `all_panes_must_have_tabs`

[mitchmindtree]

While the all_panes_must_have_tabs field works nicely with Tree::simplify, it appears to have no affect when using Tree::simplify_children_of_tile.

While working on gantz, I discovered this when attempting to use Tree::simplify_children_of_tile as a short-hand to give tab bars to all panes besides the main "workspace" area.

Also - thanks for sharing this crate!