TUI window resize should not need invisibility

TUI window resize should not need invisibility

When resizing a window, the TUI currently first makes it invisible,
then changes the size, and then restores its visibility.

I think this is done because curses doesn't truly support resizing a
window -- there is a "wresize" extension, but the man page says it
isn't available in all versions of curses.

First, this is probably not a major problem any more.  I imagine most
of those old systems are gone now.

Second, I think it's a better API to have this detail hidden inside of
the resize method.

This patch changes the code to follow this idea, and changes the
ordinary resize method to use wresize when it is available.  The
special case for the command window is also moved to methods on the
command window.

gdb/ChangeLog
2019-08-15  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (show_layout, show_source_disasm_command)
	(show_data): Don't change window visibility.
	(tui_gen_win_info::resize): Remove special case for command
	window.  Use wresize, when available.
	(show_source_or_disasm_and_command): Don't change window
	visibility.
	* tui/tui-command.h (struct tui_cmd_window) <resize>: Declare.
	<make_visible>: New method.
	* tui/tui-command.c (tui_cmd_window::resize): New method.