Allow changing the layout of workspaces by storing it (Thanks mhcerri)

Before commit 4976fa3350, setting the
layout of workspaces to something else than the default would just mess
up the parent container of the workspace (the content container).

After that commit, it would create an unnecessary split container when
you change the layout _before_ opening any containers. To avoid this, we
now store the layout (similar to how the 'workspace_layout'
configuration directive works) and apply it when the first container is
attached to the workspace.

Fixes #796

include/data.h

@@ -514,6 +514,20 @@ struct Con {
     TAILQ_HEAD(swallow_head, Match) swallow_head;
 
     enum { CF_NONE = 0, CF_OUTPUT = 1, CF_GLOBAL = 2 } fullscreen_mode;
+    /* layout is the layout of this container: one of split[v|h], stacked or
+     * tabbed. Special containers in the tree (above workspaces) have special
+     * layouts like dockarea or output.
+     *
+     * last_split_layout is one of splitv or splith to support the old "layout
+     * default" command which by now should be "layout splitv" or "layout
+     * splith" explicitly.
+     *
+     * workspace_layout is only for type == CT_WORKSPACE cons. When you change
+     * the layout of a workspace without any children, i3 cannot just set the
+     * layout (because workspaces need to be splitv/splith to allow focus
+     * parent and opening new containers). Instead, it stores the requested
+     * layout in workspace_layout and creates a new split container with that
+     * layout whenever a new container is attached to the workspace. */
     enum {
         L_DEFAULT = 0,
         L_STACKED = 1,
@@ -522,7 +536,7 @@ struct Con {
         L_OUTPUT = 4,
         L_SPLITV = 5,
         L_SPLITH = 6
-    } layout, last_split_layout;
+    } layout, last_split_layout, workspace_layout;
     border_style_t border_style;
     /** floating? (= not in tiling layout) This cannot be simply a bool
      * because we want to keep track of whether the status was set by the