ui: Fix incorrect selection of the interactive stack

Moving code for the left key to a separate function caused the key to
be considered "not handled" (`return false`) instead of
"handled" (`return true`) when at end of line.

Fixes: #1330

Signed-off-by: Christophe de Dinechin <[email protected]>

src/user_interface.cc

@@ -5650,7 +5650,7 @@ bool user_interface::do_left()
     }
 
     beep(4000, 50);
-    return false;
+    return true;
 }