Simplify TUI C-x 2 binding

Simplify TUI C-x 2 binding

The TUI "C-x 2" binding tries to switch to a different layout based on
the current layout.  Once user-defined layouts are available, this
won't really make sense.  I wasn't entirely sure how to handle this.

This patch changes the binding to simply cycle through the existing
layouts.  I considered this a reasonable, though not ideal,
compromise.

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

	* tui/tui.c (tui_rl_change_windows): Call tui_next_layout.
	* tui/tui-layout.h (tui_next_layout): Declare.
	* tui/tui-layout.c (tui_next_layout): New function.

Change-Id: Ic101f0e3831a4235a048b3090ef60f025f7449bb