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).
  *

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

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;