Make workspace_layout handle all cons at workspace level, not only the first one (+test)

This makes opening new windows on workspace level and moving windows to the
right/left more like in the old i3.

src/move.c

@@ -22,6 +22,23 @@ static void insert_con_into(Con *con, Con *target, position_t position) {
     con_detach(con);
     con_fix_percent(con->parent);
 
+    /* When moving to a workspace, we respect the user’s configured
+     * workspace_layout */
+    if (parent->type == CT_WORKSPACE) {
+        Con *split = workspace_attach_to(parent);
+        if (split != parent) {
+            DLOG("Got a new split con, using that one instead\n");
+            con->parent = split;
+            con_attach(con, split, false);
+            DLOG("attached\n");
+            con->percent = 0.0;
+            con_fix_percent(split);
+            con = split;
+            DLOG("ok, continuing with con %p instead\n", con);
+            con_detach(con);
+        }
+    }
+
     con->parent = parent;
 
     if (position == BEFORE) {
@@ -142,7 +159,8 @@ void tree_move(int direction) {
     } while (same_orientation == NULL);
 
     /* this time, we have to move to another container */
-    /* This is the container *above* 'con' which is inside 'same_orientation' */
+    /* This is the container *above* 'con' (an ancestor of con) which is inside
+     * 'same_orientation' */
     Con *above = con;
     while (above->parent != same_orientation)
         above = above->parent;