Turn tui_erase_source_content into a method

Turn tui_erase_source_content into a method

This changes tui_erase_source_content into a method on
tui_source_window_base.  The bulk of the work is moved into a helper
method, so that the callers can each pass the string appropriate to
the particular window class.

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

	* tui/tui-winsource.h (struct tui_source_window_base)
	<do_erase_source_content>: New method.
	<erase_source_content>: New method.
	(tui_erase_source_content): Don't declare.
	* tui/tui-winsource.c (tui_clear_source_content): Update.
	(tui_source_window_base::do_erase_source_content): Rename from
	tui_erase_source_content.
	(tui_source_window_base::show_source_content): Update.
	* tui/tui-win.c (tui_resize_all, tui_adjust_win_heights): Update.
	* tui/tui-source.h (struct tui_source_window)
	<erase_source_content>: New method.
	* tui/tui-disasm.h (struct tui_disasm_window)
	<erase_source_content>: New method.