Feature Request: Allow specifying paths relative to XDG user dir variables (${DOWNLOADS}/something, ${PICTURES}/something...)

[nativerv]

The problem

Currently, we can reference XDG user dirs by variables regardless of their actual name: ${DOWNLOADS}, ${DOCUMENTS}, ${PICTURES}, ${VIDEOS}, ${MUSIC}.
Many programs use these directories only to create its own directory inside and use it for its purposes.

For example:
Telegram Desktop creates and defaults to "Telegram Desktop" directory inside ${DOWNLOADS} for its downloads
Flameshot defaults to Screenshots directory inside ${PICTURES} to save its screenshots

Ideally the profiles must only whitelist these default directories to allow default behaviour and deny everything else.
But the only way to allow these dirs and not depend on any hardcoded names is to whitelist/noblacklist the entire ${DOWNLOADS}/${PICTURES}, as currently we get an error by trying to specify paths relative to XDG user dir variable:

telegram-desktop.local:

noblacklist ${DOWNLOADS}
whitelist ${DOWNLOADS}/Telegram Desktop

Reading profile /home/nrv/.config/firejail/telegram-desktop.local
Error: "${DOWNLOADS}/Telegram Desktop" is an invalid filename: rejected character: "{"

The only way to go around that right now is to hardcode directory names relative to ${HOME} variable:
telegram-desktop.local:

noblacklist ${DOWNLOADS}
whitelist ${HOME}/dl/Telegram Desktop

/etc/firejail/keepassxc.profile:

# You can enable whitelisting for keepassxc by adding the below to your keepassxc.local.
# If you do, you MUST store your database under ${HOME}/Documents/KeePassXC/foo.kdbx.
#mkdir ${HOME}/Documents/KeePassXC
#whitelist ${HOME}/Documents/KeePassXC

Describe the solution you'd like

Allow specifying paths relative to XDG user dir variables as with ${HOME} variable:

whitelist ${DOWNLOADS}/Telegram Desktop
whitelist ${PICTURES}/Screenshots
whitelist ${DOCUMENTS}/KeePassXC

Additional context

Even if we consider XDG user dirs to be "public" for use by media programs by default, one can't even change that personally without hardcoding relative to ${HOME}. What if you renamed your XDG user dirs later, e.g. Downloads -> downloads?

[rusty-snake]

Duplicate of #2359

[rusty-snake]

/etc/firejail/keepassxc.profile:

# You can enable whitelisting for keepassxc by adding the below to your keepassxc.local.
# If you do, you MUST store your database under ${HOME}/Documents/KeePassXC/foo.kdbx.
#mkdir ${HOME}/Documents/KeePassXC
#whitelist ${HOME}/Documents/KeePassXC

Even more important IMHO, #4006 requires this.