Added background and border keys to the i3bar protocol.

This patch adds two new status block keys, background and border, which
define the respective colors for the status block. If not specified, the
current behavior is kept, e.g., no background / border will be drawn.

If the status block is marked urgent, the urgent color is prioritized.

fixes #2022

i3bar/include/common.h

@@ -38,6 +38,8 @@ struct status_block {
     i3String *short_text;
 
     char *color;
+    char *background;
+    char *border;
 
     /* min_width can be specified either as a numeric value (in pixels) or as a
      * string. For strings, we set min_width to the measured text width of