Add support for adding sysroot out-of-the-box

I have a branch featuring a sysroot module here https://gitlab.gnome.org/GNOME/gnome-builder/tree/wip/tintou/sysroot I'm open to all issues that might be triggered by this branch

IdeHostSubprocessLauncher is here because I have issues with JHBuild polluting the environment

Known bug: The popover has no focus when the row is added, I couldn't get it work properly.