Better default apps

[castrojo]

Ok now that we have lived on this setup I'm making some convenience changes:

• We need flatseal to manage flatpaks so just adding it.
• File roller isn't included by default in fedora so adding that.
• Firmware is a nice GUI for fwupd and useful.
• Celluloid is based on mpv but fits nicer on the default desktop, it's playing everything I've thrown at it so good default.
• And the big one is extension manager, that looks like the stock extensions app but additionally allows search, download, and installation right from it's UI and since we can't use extensions.gnome.org right now it's a must-have. Swapping it out.

PTAL @AdamIsrael and @marcoceppi