generated xsession script shouldn't simply start window manager and then start desktop manager

[poscat0x04]

Describe the bug
Right now the generated xsession script simply starts the window manager then starts the desktop manager, this is not ideal for the following reasons:

1. If spawned separately, the processes spawned by the window manager will not enjoy the environment variables set by the DM, which is needed for app themes, etc.
2. It is fundamentally the DM's responsibility to swap out its own window manager for an external window manager and every DM does it differently, for example KDE takes an environment variable KDEWM where gnome takes a command-line argument. So I think the function that generates xsession script (xsession, in this case) should be defined in the DM's module instead of a top-level module (you could, but you'll need to distinguish DMs so that's essentially the same (something something expressions problem)).

[poscat0x04]

For some DMs it's not even possible to use an external WM.

[poscat0x04]

I recommend adding two additional attributes to DMs, the first is whether the DM is able to use an external WM, the second is a function that takes a WM and returns a string (the xsession script)

[stale]

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