core (main): add a mechanism to have a shared temp directory: this directory may be used to share files between backend and frontends. Normally, an os-dependent temporary directory is created for that, but if this option is set, the directory will be created in <local_dir>/<cache_dir>, which may be useful in some use case (e.g. containerized frontends and backend).

[Desktop Entry]
Type=Application
Name=Libervia TUI (Primitivus)
Comment=console frontend for Libervia (XMPP)
Exec=libervia-tui
Icon=org.libervia.LiberviaTUI
Categories=Network;InstantMessaging;Chat;FileTransfer;RemoteAccess;
Keywords=xmpp;chat;messaging;im;file;sharing;communication;jabber;social;network;decentralization;console;