Add preliminary printerfriendly-through-css feature. This adds a new print.css

Add preliminary printerfriendly-through-css feature. This adds a new print.css
stylesheet that is used when using the browser's print (or print preview) function.
A problem is that due to the frames, it won't work if selecting File -> Print because
this will print webmail.php instead of read_body. To see it in action, open read_body
on its own and use File -> Print Preview.

It would be nice if this could replace the printer_friendly function over time, because
it's purely css based and thus doesn't require extra code. For that we need to finetune
the sheet and most importantly, conquer the frames-problem in some smart way.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@13121 7612ce4b-ef26-0410-bec9-ea0150e637f0