thumbnail: Sandbox thumbnailers in Flatpaks

Bastien Nocera requested to merge wip/hadess/sandbox-flatpak-thumbnailer into master

Up until now, thumbnailer sandboxing was disabled in Flatpaks as bwrap cannot be used to create a namespace inside a namespace. Use Flatpak's "Spawn" method, via flatpak-spawn, to launch a separate sandbox for thumbnailing.

CC: @smcv

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