Install default Flatpak apps

[p5]

PopOS!_'s install comes with some applications pre-installed. I think we should try to match the default application lists as much as possible, but obviously manage these apps as Flatpaks.

These default apps would be good as system flatpaks, rather than user flatpaks

These apps include:

• [ ] Document Scanner
• [ ] Document Viewer
• [ ] Image Viewer
• [ ] Thunderbird (maybe leave this out since it's massive)
• [ ] Videos
• [ ] Disk Usage Analyser
• [ ] Popsicle (maybe leave this out)
• [ ] System Monitor (replace with Mission Center?)
• [ ] Character Map
• [ ] Discs
• [ ] File Roller
• [ ] Fonts
• [ ] Passwords and Keys

[p5]

Added to ISOs in #66