sideways split support for Irssi

warning:

* may be buggy

This commit adds support for sideways splits into Irssi. To that regard,
there are a number of new commands available

the "R" commands:
	/window new rsplit - make a new sideways split
	/window rshow - show an existing window to the right
	/window rgrow/rshrink/rsize/rbalance
	 - manipulate the size of your sideways split windows

the "D" commands:
	/window dup/ddown/dleft/dright
	 - navigate the windows directionally, as an alternative to
	   /window up/down that you can bind to some key
	/window move dleft/dright - the same for moving

Enjoy!

themes/default.theme

@@ -251,6 +251,7 @@ abstracts = {
   # default background for all statusbars. You can also give
   # the default foreground color for statusbar items.
   sb_background = "%4%w";
+  window_border = "%4%w";
 
   # default backround for "default" statusbar group
   #sb_default_bg = "%4";