Confirm closing tabs

This MR adds support for making DBus calls to the Flatpak service. This allows us to do all sorts of useful things — forwarding FDs and reading process information, for example.

On top of that, this MR also makes use of this new capability by:

• Getting the real* PID of the default process running in the terminal
• Getting the PID of the foreground process running in the terminal (this will be the shell or any other command the user may be running)
• Prompting the user to confirm before closing a tab that still has a running process
• Prompting the user to confirm before closing a Window with one or many tabs that still have a running process
• All of this works in non-flatpak as well

*Real PID means the PID on the host, which is different than the PID in the sandbox

Preview