Question List Mode implementation

This uses tabulated-list-mode to build the list of questions.
The actual content of questions is displayed on a separate buffer using org-mode (for now of course, this is not permanent).
`tabulated-list-mode` needs a column to sort by. We have to manually write the sorting function we want, so the column is completely irrelevant, but it still demands that we give it a column.

Everything on the "second line" in the questions list (the date and the tags) at the moment, isn't really a field of its own. Since "Date" doesn't have an entry at the header-line, there's no “Date” column. Since it also makes no useful sense to sort by title here, that's the solution I found.

We can improve this situation by calling the third column "Details" (or hell, just "Date"). On the other hand, I remember we've been discussing giving the beaderline a completely different purpose as well.