Bundles which require a WM should be avoided in headless environments

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`.
