Use Firejail if available

[James]

Since pipelight-plugin is used to run closed-source browser plugins on an otherwise open-source system, it would be a very desirable feature for pipelight-plugin to use Firejail ( https://firejail.wordpress.com/ ) for sandboxing these closed-source plugins if Firejail is present on the system at runtime.

Firejail has almost no overhead, does not require any setup or system changes beyond the installation of a debian file, and seems to provide effective sandboxing. Having Firejail integration in Pipelight would allow many users who may feel uncomfortable allowing closed-source code to run even in Wine to use these closed-source plugins in appropriate security without going through the hassle and overhead of a VM.