Add tab completion for TUI's "focus" command

Add tab completion for TUI's "focus" command

The implementation is pretty straightforward, with the only caveat being
that the "src", "cmd", "next" and "prev" entries get delibrately added
to the completion list even when the TUI has not yet been initialized
(i.e. has never been enabled during the session), since invoking the
"focus" command with these arguments already works when the TUI has not
yet been initialized.

gdb/ChangeLog:

	* tui/tui-win.c (focus_completer): New static function.
	(_initialize_tui_win): Set the completion function of the
	"focus" command to focus_completer.

gdb/testsuite/ChangeLog:

	* gdb.base/completion.exp: Test the completion of the "focus"
	command.