Introduce splith/splitv layouts, remove orientation

With this commit, the "default" layout is replaced by the splith and
splitv layouts. splith is equivalent to default with orientation
horizontal and splitv is equivalent to default with orientation
vertical.

The "split h" and "split v" commands continue to work as before, they
split the current container and you will end up in a split container
with layout splith (after "split h") or splitv (after "split v").

To change a splith container into a splitv container, use either "layout
splitv" or "layout toggle split". The latter command is used in the
default config as mod+l (previously "layout default"). In case you have
"layout default" in your config file, it is recommended to just replace
it by "layout toggle split", which will work as "layout default" did
before when pressing it once, but toggle between horizontal/vertical
when pressing it repeatedly.

The rationale behind this commit is that it’s cleaner to have all
parameters that influence how windows are rendered in the layout itself
rather than having a special parameter in combination with only one
layout. This enables us to change existing split containers in all cases
without breaking existing features (see ticket #464). Also, users should
feel more confident about whether they are actually splitting or just
changing an existing split container now.

As a nice side-effect, this commit brings back the "layout toggle"
feature we once had in i3 version 3 (see the userguide).

AFAIK, it is safe to use in-place restart to upgrade into versions
after this commit (switching to an older version will break your layout,
though).

Fixes #464

docs/ipc

@@ -1,7 +1,7 @@
 IPC interface (interprocess communication)
 ==========================================
 Michael Stapelberg <michael@i3wm.org>
-July 2012
+August 2012
 
 This document describes how to interface with i3 from a separate process. This
 is useful for example to remote-control i3 (to write test cases for example) or
@@ -270,12 +270,15 @@ border (string)::
 	Can be either "normal", "none" or "1pixel", dependending on the
 	container’s border style.
 layout (string)::
-	Can be either "default", "stacked", "tabbed", "dockarea" or "output".
+	Can be either "splith", "splitv", "stacked", "tabbed", "dockarea" or
+	"output".
 	Other values might be possible in the future, should we add new
 	layouts.
 orientation (string)::
 	Can be either "none" (for non-split containers), "horizontal" or
 	"vertical".
+	THIS FIELD IS OBSOLETE. It is still present, but your code should not
+	use it. Instead, rely on the layout field.
 percent (float)::
 	The percentage which this container takes in its parent. A value of
 	+null+ means that the percent property does not make sense for this