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

i3.config.keycodes

@@ -69,7 +69,7 @@ bindcode $mod+43 split h
 bindcode $mod+55 split v
 
 # enter fullscreen mode for the focused container
-bindcode $mod+41 fullscreen
+bindcode $mod+41 fullscreen toggle
 
 # change container layout (stacked, tabbed, toggle split)
 bindcode $mod+39 layout stacking