Bundles which require a WM should be avoided in headless environments

[elamperti]

Bundles should have an optional requires_wm() function, making the exit code be 0 if it requires a window manager. It may explicitly return 1 to specify it doesn't require a WM, but leaving the function undeclared will make it exit with code 127 (command not found). Considering this, the implementation should expect a window manager if and only if the return code is 0.