Rewrite the split and merge tools

[vkbo]

The split and merge tool need to be rewritten to support the updated project structure. There are also some usability issues with the current tool. See #1032.

A new split/merge tool could include the feature #1071.

The new tool could be implemented as a set of conversion tools with multiple options:

• Convert folder to document
• Convert parent item with child items into a single document (equivalent to current merge tool)
• Convert a single document to a document or folder with child documents (equivalent to current split tool)
• Replace a document with a folder, which moves the document to Trash and inserts a folder where it used to be, updating the child items with a new parent.