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/data.h

@@ -75,6 +75,12 @@ typedef enum { ADJ_NONE = 0,
                ADJ_UPPER_SCREEN_EDGE = (1 << 2),
                ADJ_LOWER_SCREEN_EDGE = (1 << 4) } adjacent_t;
 
+typedef enum { HEBM_NONE = ADJ_NONE,
+               HEBM_VERTICAL = ADJ_LEFT_SCREEN_EDGE | ADJ_RIGHT_SCREEN_EDGE,
+               HEBM_HORIZONTAL = ADJ_UPPER_SCREEN_EDGE | ADJ_LOWER_SCREEN_EDGE,
+               HEBM_BOTH = HEBM_VERTICAL | HEBM_HORIZONTAL,
+               HEBM_SMART = (1 << 5) } hide_edge_borders_mode_t;
+
 typedef enum { MM_REPLACE,
                MM_ADD } mark_mode_t;