Don't erase TUI source window when switching focus

Don't erase TUI source window when switching focus

PR tui/26719 points out that switching the focus can erase the TUI
source window.  This is a regression introduced by the patch to switch
the source window to using a pad.

This patch fixes the bug by arranging to call prefresh whenever the
window is refreshed.

2020-10-19  Tom Tromey  <tromey@adacore.com>

	PR tui/26719
	* tui/tui-winsource.h (struct tui_source_window_base)
	<refresh_window>: Rename from refresh_pad.
	* tui/tui-winsource.c (tui_source_window_base::refresh_window):
	Rename from refresh_pad.
	(tui_source_window_base::show_source_content)
	(tui_source_window_base::do_scroll_horizontal): Update.

gdb/testsuite/ChangeLog
2020-10-19  Tom Tromey  <tromey@adacore.com>

	PR tui/26719
	* gdb.tui/list.exp: Check source window contents after focus
	change.