Add flathub token file

It's so that apps put on flathub are marked as verified. See https://docs.flathub.org/docs/for-app-authors/verification

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@skunnyk In what flathub asks me, the file is supposed to appear at this address: https://xfce.org/.well-known/org.flathub.VerifiedApps.txt

I hope it's ok, but basically I'm not sure what I'm doing :)

Motivation:

• unverified apps tend to be hidden by default in distribution app stores, as is now the case with Linux Mint: https://blog.linuxmint.com/?p=4719
• also, the app has a warning badge on flathub: https://flathub.org/apps/org.xfce.mousepad

See also:

• https://github.com/flathub/org.xfce.mousepad/issues/58
• https://github.com/flathub/org.xfce.ristretto/issues/123