Reimplement TUI "C-x 1" binding

Reimplement TUI "C-x 1" binding

The TUI "C-x 1" key binding removes TUI windows, based on the current
layout.  With user-defined layouts, this is no longer easy to do.

This patch changes "C-x 1" to simply delete windows, leaving just the
focus window, the locator, and the command window.

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

	* tui/tui.c (tui_rl_delete_other_windows): Call
	tui_remove_some_windows.
	* tui/tui-layout.h (class tui_layout_base) <remove_windows>:
	Declare method.
	(class tui_layout_window) <remove_windows>: New method.
	(class tui_layout_split) <remove_windows>: Declare.
	(tui_remove_some_windows): Declare.
	* tui/tui-layout.c (tui_remove_some_windows): New function.
	(tui_layout_split::remove_windows): New method.

Change-Id: If186f9c3f263913e963b965204481d1b4385c6d4