Allow merging of files at some path relative to the root

[abraemer]

It is conceivable that some parts of the file tree either have information due to previous work (e.g. code that didn't change) or was scanned with a different scanner or similar. In such a case, one needs the option to not just merge to opossum files but also specify a relative location where one should be integrated into the other.
This reflects the relative merge operation from opossum.lib.hs (yet to be implemented in opossum-file: see opossum-tool/opossum-file#55).

They way this could be implemented in OpossumUI is by using a new context menu (aka right-click menu) in the file tree. This context menu allows the user to select the desired merge directly:

Selecting the Menu point (tentatively named "Import as subfolder...") opens up the usual import dialog (restricted to opossum files?) and then performs the merge-relative operation behind the scenes integrate the new information into the existing document.

Things to consider:

• Is the name of the subfolder always clear? Do we need an option for the user change it? I think currently for all opossum files "in nature", the root folder "/" always has a single child. Thus the new structure is always clear. However I don't think this guaranteed in principle and just caused by the methods of construction (e.g. ScanCode always has a single root directory). If the root has multiple children, do we just put them beneath the selected folder?
• Related to above: Do we want to allow adding new subfolders to "/"?
• Should this merge never "create" new directories but rather replace the selected one? This could also resolve the above issue
• What if the user has unsaved changes? We could probably just show the same popup that we used for Open/Import/Export. See Show save dialog before open/import/merge/export #2775
• Are there other - perhaps simpler - solutions to this workflow?
  • Option for import: Risks making the import more complicated for a function considered for expert users. Specifying the merge location seems difficult and would require some thinking
  • Standalone menu entry: Menu gets longer and longer. Same issue with specifying the merge location
  • Button somewhere else in the UI? Conceptually the place for file operations is the file tree. Maybe there could be a Toolbar between the Header "All Resources" and the actual tree?