mutt: improve <space> binding in index views
mutt: improve <space> binding in index views

We already have <enter> for <display-message>; let's use <space>
consistently with how we do in the pager view (ie. it should scroll the
principal view down).

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent