Don't let TUI focus on locator

Don't let TUI focus on locator

PR tui/26638 notes that the C-x o binding can put the focus on the
locator window.  However, this is not useful and did not happen
historically.  This patch changes the TUI to skip this window when
switching focus.

gdb/ChangeLog
2020-09-24  Tom Tromey  <tromey@adacore.com>

	PR tui/26638:
	* tui/tui-stack.h (struct tui_locator_window) <can_focus>: New
	method.
	* tui/tui-data.h (struct tui_win_info) <can_focus>: New method.
	* tui/tui-data.c (tui_next_win): Exclude non-focusable windows.
	(tui_prev_win): Rewrite.

gdb/testsuite/ChangeLog
2020-09-24  Tom Tromey  <tromey@adacore.com>

	PR tui/26638:
	* gdb.tui/list.exp: Check output of "focus next".