When in tabbed mode, nail the orientation to horizontal

This is intuitive for the user as he can use the left/right keys to switch.
Also, we need to nail the orientation because you can be in either vertical or
horizontal mode when you enter the tabbed layout (like with the stacking
layout).

src/con.c

@@ -408,6 +408,9 @@ int con_orientation(Con *con) {
     if (con->layout == L_STACKED)
         return VERT;
 
+    if (con->layout == L_TABBED)
+        return HORIZ;
+
     return con->orientation;
 }