Implements a PGF Key for page layout; adapts tutorial

In accordance to what was discussed in #3, this implements a PGF Key to steer the multi page layout, `page layout`. Per default, it is not used. When it is used, the option to the key is directly passed to `\pgfpagesuselayout{OPT}`, for example `\fzjset{page layout=2 on 1}`.

Since the page layout is actually different from handout, I removed the `\mode<handout>` part from the key's definition. Users should rather wrap the `\fzjset` call in the `\mode<handout>` macro.

The tutorial has been changed to reflect (and explain) that; the minimal_handout.tex file changed as well.
1 job for feature--page-layout