Implement "trashcan"

[mkoskim]

To remove entire scenes, chapters and acts, implement a trashcan section in the same way it was implemented in ancient moe editor: it is a section that is not written to file, so everything it contains will be trashed when reloading the file. We might consider another way: save trashcan to file, but give an option to empty it. And of course, one way to do this is to implement context menu with remove option.