plugin app manager: add a way to create files: a `files` field can now be used to create files on the fly. Its value must be a dict where key are file names, and values are an other dict with file data. For now file data can only use the `content` key. Value of content will be written to the file. If a file already exists, it is not overwritten.

#!/bin/sh

eval $(dbus-launch --sh-syntax)

nohup /usr/bin/Xtigervnc -geometry 1024x768 -desktop "Libervia e2e test" -rfbport 5900 -SecurityTypes None :0 &
nohup openbox &
exec libervia-backend fg