Make "scratchpad show" return correct info

Fix the issue #3227(https://github.com/i3/i3/issues/3227).

1).Make cmd_scratchpad_show() use the information coming from scratchpad_show().
2).Add a test case 298-scratchpad-show.t.

include/scratchpad.h

@@ -29,7 +29,7 @@ void scratchpad_move(Con *con);
  * can press the same key to quickly look something up).
  *
  */
-void scratchpad_show(Con *con);
+bool scratchpad_show(Con *con);
 
 /**
  * When starting i3 initially (and after each change to the connected outputs),