Rename private data members of tui_data_window

Rename private data members of tui_data_window

As Pedro suggested, this patch renames the private data members of
tui_data_window to use the "m_" prefix.

gdb/ChangeLog
2019-09-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.h (struct tui_data_window) <get_current_group>:
	Update.
	<m_regs_content, m_regs_column_count, m_current_group>: Add "m_"
	prefix.
	* tui/tui-regs.c (tui_data_window::last_regs_line_no)
	(tui_data_window::line_from_reg_element_no)
	(tui_data_window::first_reg_element_no_inline)
	(tui_data_window::show_registers)
	(tui_data_window::show_register_group)
	(tui_data_window::display_registers_from)
	(tui_data_window::display_registers_from_line)
	(tui_data_window::first_data_item_displayed)
	(tui_data_window::delete_data_content_windows)
	(tui_data_window::erase_data_content)
	(tui_data_window::do_scroll_vertical)
	(tui_data_window::refresh_window)
	(tui_data_window::check_register_values): Update.