Remove hard-coded TUI layouts

Remove hard-coded TUI layouts

This changes the TUI so that the available layouts are no longer
completely hard-coded.  "enum tui_layout_type" is removed, and then
all the fallout from this is fixed up.

This patch also reimplements the "layout" command to be a prefix
command.  The concrete layouts are simply sub-commands now.  This
provides completion and correct abbreviation behavior for free.

Finally, this also changes the name of the locator window to "status".
This matches the documentation and will be exposed to the user in a
subsequent patch.

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

	* tui/tui.c (tui_enable): Call tui_set_initial_layout.
	* tui/tui-win.c (window_name_completer): Update comment.
	* tui/tui-layout.h (class tui_layout_base) <replace_window>:
	Declare method.
	(class tui_layout_window) <replace_window>: Likewise.
	(class tui_layout_split) <replace_window>: Likewise.
	(tui_set_layout): Don't declare.
	(tui_set_initial_layout): Declare function.
	* tui/tui-layout.c (layouts, applied_skeleton, src_regs_layout)
	(asm_regs_layout): New globals.
	(tui_current_layout, show_layout): Remove.
	(tui_set_layout, tui_add_win_to_layout): Rewrite.
	(find_layout, tui_apply_layout): New function.
	(layout_completer): Remove.
	(tui_next_layout): Reimplement.
	(tui_next_layout_command): New function.
	(tui_set_initial_layout, tui_prev_layout_command): New functions.
	(tui_regs_layout): Reimplement.
	(tui_regs_layout_command): New function.
	(extract_display_start_addr): Rewrite.
	(next_layout, prev_layout): Remove.
	(tui_layout_window::replace_window): New method.
	(tui_layout_split::replace_window): New method.
	(destroy_layout): New function.
	(layout_list): New global.
	(add_layout_command): New function.
	(initialize_layouts): Update.
	(tui_layout_command): New function.
	(_initialize_tui_layout): Install "layout" commands.
	* tui/tui-data.h (enum tui_layout_type): Remove.
	(tui_current_layout): Don't declare.

Change-Id: I9b5f7ab3ce838d6b340b8c373ef649a8e0a74b73