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/config.h

@@ -125,7 +125,7 @@ struct Config {
      * This is useful if you are reaching scrollbar on the edge of the
      * screen or do not want to waste a single pixel of displayspace.
      * By default, this is disabled. */
-    adjacent_t hide_edge_borders;
+    hide_edge_borders_mode_t hide_edge_borders;
 
     /** By default, a workspace bar is drawn at the bottom of the screen.
      * If you want to have a more fancy bar, it is recommended to replace