Change tui_show_source_content to be a method

Change tui_show_source_content to be a method

This changes tui_show_source_content to be a method on
tui_source_window_base.

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

	* tui/tui-winsource.h (struct tui_source_window_base)
	<show_source_content>: Declare.
	(tui_show_source_content): Don't declare.
	* tui/tui-winsource.c (tui_update_source_window_as_is): Update.
	(tui_source_window_base::show_source_content): Rename from
	tui_show_source_content.
	(tui_source_window_base::refresh_all): Update.
	* tui/tui-layout.c (show_source_disasm_command)
	(show_source_or_disasm_and_command): Update.