Use /usr/local/share/xsessions

Right now we are not using `/usr/local/etc/X11/xinit/` and `/usr/local/share/xsessions`; instead we have `login_cmd exec start-hello` hardcoded in `/usr/local/etc/slim.conf`. This is clearly not ideal since it does not allow users to install additional desktops.

So far, I could not figure out what exactly I have to put in `login_cmd` so that the desktop file in `/usr/local/share/xsessions/*.desktop` gets read and the command listed under `Exec=` there gets executed.

If possible, I would like to avoid that the user needs a file in `~/.xinitrc` because if the user deletes that, it would break the graphical desktop, which is very undesirable.

In slim, no matter what I tried so far, I get "Failed to execute login command".

Isn't slim capable of reading the desktop files? Maybe we should have another look at the Lumina login thing. Especially since it is written in Qt like the rest of the hello desktop.

cc @GrahamPerrin