Separator color via config; separator width and on/off via ipc

This patch adds the following features:
1) Configure a color of the separator via config.  It is done like
   bar {
      colors {
         separator #000000
      }
   }
2) A block can have an integer entry "separator_block_width" which
   sets the width of the gap which would follow after the current block.

3) A block can have a boolean entry "separator" and if it is set
   to false, then the drawing of the separating line would be disabled.

i3bar/include/xcb.h

@@ -28,6 +28,7 @@
 struct xcb_color_strings_t {
     char *bar_fg;
     char *bar_bg;
+    char *sep_fg;
     char *active_ws_fg;
     char *active_ws_bg;
     char *active_ws_border;