Refactor question list

There's no longer a `list-questions` command. Instead, we have a `sx-tab-frontpage` command.

There's also a `sx-tab--define` function, which will greatly simplify defining new tabs.
It defines a command which creates a buffer in `sx-question-list-mode` and sets up to 3 variables necessary for the mode to function.
To know how to use these variables, read their docs and the doc of `sx-question-list-mode`.

I'll try to update sx.org later this week. If I have time.
Possibly, I just couldn't decide on a good to way to do that. 
I mean, does it make sense to move down 5 entries and then visit? what are the odds you'll want to visit the 5th entry from now, but not the 4th? 

Or maybe I'm just putting too much thought into something silly. :-)
In any case, I'd leave this for later anyway.