Flatpak for monogram tool

[sfkleach]

The advantage of using Flatpak is that we can sandbox monogram, which should be reassuring to nervous sysadmins, and hopefully get it into the Linux software package ecosystem.

• Get the sandboxing running
• Pick the right options so that the -i and -o options work
• Integrate into the build system
• Add to Flathub
• Make the directory entry on Software Manager look good???