Store aspect_ratio instead of weird proportional_{width,height} (Thanks phillip)

This commit only goes to “next” because I am not sure whether it
actually makes things better in all cases and want to give it some
testing first.

There was no documented reason behind using the
proportional_{width,height} variables, so I suppose that code was just
stupidity on my part (it was written merely a month after I started this
project in 2009).

fixes #1032

src/con.c

@@ -51,6 +51,7 @@ Con *con_new_skeleton(Con *parent, i3Window *window) {
     Con *new = scalloc(sizeof(Con));
     new->on_remove_child = con_on_remove_child;
     TAILQ_INSERT_TAIL(&all_cons, new, all_cons);
+    new->aspect_ratio = 0.0;
     new->type = CT_CON;
     new->window = window;
     new->border_style = config.default_border;