Introduce tui_data_window::erase_data_content method

Introduce tui_data_window::erase_data_content method

This changes tui_erase_data_content to be a method on tui_data_window,
allowing for the removal of some uses of the TUI_DATA_WIN global.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-windata.h (tui_erase_data_content): Don't declare.
	* tui/tui-windata.c (tui_data_window::erase_data_content): Rename
	from tui_erase_data_content.
	(tui_data_window::display_all_data)
	(tui_data_window::refresh_all)
	(tui_data_window::do_scroll_vertical): Update.
	* tui/tui-regs.c (tui_show_registers): Update.
	* tui/tui-data.h (struct tui_data_window) <erase_data_content>:
	New method.