Use a curses pad for source and disassembly windows

Use a curses pad for source and disassembly windows

This changes the TUI source and disassembly windows to use a curses
pad for their text.  This is an important step toward properly
handling non-ASCII characters, because it makes it easy to scroll
horizontally without needing gdb to also understand multi-byte
character boundaries -- this can be wholly delegated to curses.
Horizontal scrolling is probably also faster now, because no
re-rendering is required.

gdb/ChangeLog
2020-09-27  Tom Tromey  <tom@tromey.com>

	* unittests/tui-selftests.c: Update.
	* tui/tui-winsource.h (struct tui_source_window_base)
	<extra_margin, show_line_number, refresh_pad>: New methods.
	<m_max_length, m_pad>: New members.
	(tui_copy_source_line): Update.
	* tui/tui-winsource.c (tui_copy_source_line): Remove line_no,
	first_col, line_width, ndigits parameters.  Add length.
	(tui_source_window_base::show_source_line): Write to pad.  Line
	number now 0-based.
	(tui_source_window_base::refresh_pad): New method.
	(tui_source_window_base::show_source_content): Write to pad.  Call
	refresh_pad.
	(tui_source_window_base::do_scroll_horizontal): Call refresh_pad,
	not refill.
	(tui_source_window_base::update_exec_info): Call
	show_line_number.
	* tui/tui-source.h (struct tui_source_window) <extra_margin>: New
	method.
	<m_digits>: New member.
	* tui/tui-source.c (tui_source_window::set_contents): Set m_digits
	and m_max_length.
	(tui_source_window::show_line_number): New method.
	* tui/tui-io.h (tui_puts): Fix comment.
	* tui/tui-disasm.c (tui_disasm_window::set_contents): Set
	m_max_length.