[Feature]: Recursive import of Folders

[DavidLazarescu]

Description

Import whole folders from the filesystem into Librum.

Additional Information

• Will probably need to make it an option in the UI, since Qt doesn't offer a dialog that allows selecting both folders and files.

• Optionally add them to a folder in the application which has the default name of the folder in the filesystem but is modifiable.

• Since we don't have folders inside of folders in Librum, files in subfolders of the imported folder should simply be added in a flat manner.