Move current_layout to tui-layout.c

Move current_layout to tui-layout.c

This moves the current_layout global to tui-layout.c.  This allows for
the removal of an accessor function; but also it just seems clearer to
have it here.

gdb/ChangeLog
2019-08-13  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (current_layout, tui_current_layout): Move from
	tui-data.c.
	(show_source_disasm_command, show_data)
	(show_source_or_disasm_and_command): Don't use
	tui_set_current_layout_to.
	* tui/tui-data.h (tui_set_current_layout_to): Don't declare.
	* tui/tui-data.c (current_layout, tui_current_layout): Move to
	tui-layout.c.
	(tui_set_current_layout_to): Remove.