This patch adds the ability to go to the next nonquote block of text while reading a mail.

problems i see :
 - it starts from the begining, even if you already scrolled into the mail. it's like the "search" thing. not easy way around ?
 - it doesnt take [...] as quotes, maybe there is a clean way to do it using gtk tags?
 - maybe the work shouldn't be in gtkutils, like its not really gtk-related. but then it might be messier to do ...
 - maybe the 'b' key should be listed in a menu ? and then add ability to change the key ? but 'enter' or 'space' or 'a' are not in menus, they are hardcoded
 - how does the user know about this key ?