Allow TUI sub-layouts in "new-layout" command

The new TUI layout engine has support for "sub-layouts" -- this is a
layout that includes another layout as a child.  A sub-layout is
treated as a unit when allocating space.

There's not a very strong reason to use sub-layouts currently.  This
patch exists to introduce the idea, and to simplify the subsequent
patch that adds horizontal layouts -- where sub-layouts are needed.

Because this patch won't go in on its own, I chose to defer
documenting this change until the subsequent horizontal layout patch.

gdb/ChangeLog
2020-02-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.h (class tui_layout_split) <add_split>: Change
	parameter and return types.
	(class tui_layout_base) <specification>: Add "depth".
	(class tui_layout_window) <specification>: Add "depth".
	(class tui_layout_split) <specification>: Add "depth".
	* tui/tui-layout.c (tui_layout_split::add_split): Change parameter
	and return types.
	(tui_new_layout_command): Parse sub-layouts.
	(_initialize_tui_layout): Update help string.
	(tui_layout_window::specification): Add "depth".
	(add_layout_command): Update.

gdb/testsuite/ChangeLog
2020-02-22  Tom Tromey  <tom@tromey.com>

	* gdb.tui/new-layout.exp: Add sub-layout tests.

Change-Id: Iddf52d067a552c168b8a67f29caf7ac86404b10c