Smart option added to hide_edge_borders config param (#2191) (#2191)

Use case:

* When managing multiple terminals in a workspace, the borders makes it easier
to know where the focus is, but when there is only one it's obvious where the
focus is.

* When there's only a web browser for example, the borders are actually counter-
productive since it makes clicking a side scrollbar or a tab a bit harder (if I
smash my cursor to the side or the top of the workspace, I have to move it in
the other direction by just a few pixels to be able to grab it)

Behaviour:

* No borders when there's a single window in a workspace
* Borders when there are multiple windows in a workspace

fixes #2188

include/con.h

@@ -200,6 +200,13 @@ Con *con_for_window(Con *con, i3Window *window, Match **store_match);
  */
 int con_num_children(Con *con);
 
+/**
+ * Returns the number of visible non-floating children of this container.
+ * For example, if the container contains a hsplit which has two children,
+ * this will return 2 instead of 1.
+ */
+int con_num_visible_children(Con *con);
+
 /**
  * Count the number of windows (i.e., leaf containers).
  *