Links to external documents

[peter88213]

Currently (v4.42), images can be linked to characters, locations, and items. This shall be replaced by general links.

• There shall be a folding frame called "Links" containing a list of links (label + file path or URL).
• In the novelyst settings, there is an editable list of file extensions and their associated application launchers.
• Linked files with extensions that are not listed, are opened with the system's standard application.

This allows for deep-linking e.g. Zim wiki pages.