Use an inner window in tui_py_window

Use an inner window in tui_py_window

This changes tui_py_window to create an inner curses window.  This
greatly simplifies tui_py_window::output, beacuse it no longer needs
to be careful to avoid overwriting the window's border.  This patch
also makes it a bit easier for a later patch to rewrite
tui_copy_source_line.

gdb/ChangeLog
2020-09-27  Tom Tromey  <tom@tromey.com>

	* python/py-tui.c (class tui_py_window) <refresh_window>: New
	method.
	<erase>: Update.
	<cursor_x, cursor_y>: Remove.
	<m_inner_window>: New member.
	(tui_py_window::rerender): Create inner window.
	(tui_py_window::output): Write to inner window.