Modify the pager to use windows.

Modify the pager to use windows.

The pager view is somewhat complex, due to the "mini-index" and the
positioning of the index_status vs pager_status changes depending on
$status_on_top as well as $pager_index_lines and $help.

To make things simpler, it creates four "pager windows":
index_status_window, index_window, pager_status_window, pager_window.

mutt_term_width() is renamed to mutt_window_wrap_cols() and a
mutt_window_t parameter passed in.

Also, the pager drawing loop calling display_line() has a
mutt_window_move() added to move to the beginning of the next line
each time.  This is in case the pager_window isn't flush with the left
side of the terminal.