Add the "tui new-layout" command

Add the "tui new-layout" command

This adds a new command, "tui new-layout".  This command can be used
to define a new TUI window layout.

The command is used like:

(gdb) tui new-layout name src 1 regs 1 status 0 cmd 1

The first argument is the name of the layout.  In this example, it is
"name", so the new layout could be seen by "layout name".

Subsequent arguments come in pairs, where the first item in a pair is
the name of a window, and the second item in a pair is the window's
weight.  A weight is just an integer -- a window's allocated size is
proportional to the total of the weights given.  So, in the above
example, all windows will have the same size (the status windows's
weight does not matter, because it has fixed height).

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

	* NEWS: Add "tui new-layout" item.
	* tui/tui-layout.c (add_layout_command): Return cmd_list_element.
	Add new-layout command to help text.
	(validate_window_name): New function.
	(tui_new_layout_command): New function.
	(_initialize_tui_layout): Register "new-layout".
	(tui_layout_window::specification): New method.
	(tui_layout_window::specification): New method.
	* tui/tui-layout.h (class tui_layout_base) <specification>: New
	method.
	(class tui_layout_window) <specification>: New method.
	(class tui_layout_split) <specification>: New method.

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

	* gdb.texinfo (TUI Overview): Mention user layouts.
	(TUI Commands): Document "tui new-layout".

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

	* gdb.tui/new-layout.exp: New file.

Change-Id: Id7c3ace20ab1e8924f8f4ad788f40210f58a5c05