[RP] Window list mechanism updates

[shawn]

So I've updated how ratpoison keeps track of windows. There are now 2
lists: the mapped window list and the unmapped window list. When a
window is mapped it is moved from the unmapped window list to the
mapped window list and inserted into its sorted position based on its
window number. Anyway, this impacted a lot of stuff so let me know if
it crashes or breaks anything. I punted around with emacs, rxvt,
netscape, gimp and xfig windows and noted that things worked pretty
well.

You'll also notice that there is no longer rp_window_head and
rp_window_tail. They have been replaced with rp_mapped_window_sentinel
and rp_unmapped_window_sentinel. The head of the mapped window list is
now obtained like this:

rp_mapped_window_sentinel->next

and the tail:

rp_mapped_window_sentinel->prev

the same goes for the unmapped window list. rp_current_window
functions the same way. There is a comment about these sentinels in
data.h if you want more information.

Eventually there will be 3 lists: Normal windows, Withdrawn windows,
and Iconified windows. But that is for another day...