Extend the fullscreen command

Rather than just toggling the fullscreen modes, allow to set them
directly with:

    fullscreen enable|toggle [global]
    fullscreen disable

For compatibility, retain the previous command and its toggling behavior:

    fullscreen [global]

fixes #1120

man/i3.man

@@ -230,7 +230,7 @@ bindsym Mod1+h split h
 bindsym Mod1+v split v
 
 # enter fullscreen mode for the focused container
-bindsym Mod1+f fullscreen
+bindsym Mod1+f fullscreen toggle
 
 # change container layout (stacked, tabbed, default)
 bindsym Mod1+s layout stacking