Make some tui_source_window_base members "protected"

Make some tui_source_window_base members "protected"

This renames a few members of tui_source_window_base, and makes them
"protected".

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

	* tui/tui-layout.c (extract_display_start_addr): Rewrite.
	* tui/tui-disasm.h (struct tui_disasm_window)
	<display_start_addr>: Declare.
	* tui/tui-source.h (struct tui_source_window)
	<display_start_addr>: Declare.
	* tui/tui-winsource.h (struct tui_source_window_base)
	<show_source_line, display_start_addr>: New methods.
	<m_horizontal_offset, m_start_line_or_addr, m_gdbarch, m_content>:
	Rename and move to protected section.
	* tui/tui-winsource.c (tui_source_window_base::update_source_window)
	(tui_source_window_base::do_erase_source_content): Update.
	(tui_source_window_base::show_source_line): Now a method.
	(tui_source_window_base::show_source_content)
	(tui_source_window_base::tui_source_window_base)
	(tui_source_window_base::rerender)
	(tui_source_window_base::refill)
	(tui_source_window_base::do_scroll_horizontal)
	(tui_source_window_base::set_is_exec_point_at)
	(tui_source_window_base::update_breakpoint_info)
	(tui_source_window_base::update_exec_info): Update.
	* tui/tui-source.c (tui_source_window::set_contents)
	(tui_source_window::showing_source_p)
	(tui_source_window::do_scroll_vertical)
	(tui_source_window::location_matches_p)
	(tui_source_window::line_is_displayed): Update.
	(tui_source_window::display_start_addr): New method.
	* tui/tui-disasm.c (tui_disasm_window::set_contents)
	(tui_disasm_window::do_scroll_vertical)
	(tui_disasm_window::location_matches_p): Update.
	(tui_disasm_window::display_start_addr): New method.

Change-Id: I74d72b9da5f458664427db643a108634690c6e19