Split/Extract subdirectories as standalone opossum files

[abraemer]

It could be useful to extract a whole subtree of the currently open opossum file to either work on separately or store separately for later use.
This is kind of the inverse operation to #2776 but would not depend on opossum-file to do the actual work. In combination with #2776 this opens up a whole new range of possible workflows:

• Multiple person could work on the same project simultaneously by first splitting it up and then merging the parts back together
• One could save work by extracting partial information from previous versions of a project

This should be implemented in a similar manner as #2776 (e.g. another option in the same context menu). Selecting the option then opens up a save dialog (which can likely be reused from elsewhere in the code) to save the new opossum file.

Things to consider:

• use opossum-file to implement the extraction and call that from OpossumUI or implement the splitting logic in OpossumUI directly? I like the conceptual idea that opossum-file supports the same "high-level" operations on opossum files since it could allow for more automation. However this requires that opossum-file learns more about the "output.json" part of opossum files, so it might be easier to implement in OpossumUI
• In case there are unsaved changed, we probably want to show the saving dialog again Show save dialog before open/import/merge/export #2775
• Should we "delete" the extracted folder? Either by removing the whole folder itself or at least all children? This could enable the re-merging of the extracted opossum. Alternatively we could add other editing tools for the file structure (see New workflow ideas for OpossumUI based on a context menu in the file tree #2777)
• Should we disallow "extracting" the root folder "/"?