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

parser-specs/config.spec

@@ -122,10 +122,10 @@ state NEW_WINDOW_PIXELS_PX:
   end
       -> call cfg_new_window($windowtype, $border, &width)
 
-# hide_edge_borders <none|vertical|horizontal|both>
+# hide_edge_borders <none|vertical|horizontal|both|smart>
 # also hide_edge_borders <bool> for compatibility
 state HIDE_EDGE_BORDERS:
-  hide_borders = 'none', 'vertical', 'horizontal', 'both'
+  hide_borders = 'none', 'vertical', 'horizontal', 'both', 'smart'
       -> call cfg_hide_edge_borders($hide_borders)
   hide_borders = '1', 'yes', 'true', 'on', 'enable', 'active'
       -> call cfg_hide_edge_borders($hide_borders)