Several improvements for subtopics format

1. sometimes, we need to merge several subtopics forward into the next one. But xmind didn't provide any functions to do so.

Now I use the method below(add a summary and make new subtopics with the summary) to finish that. But the overlap is there and the boundary didn't extend automatically.

Please add a new format to merge several subtopics to generate a new sub-subtopic. Thanks!