Implement showing window icons in titlebar (#4439)

This feature defaults to off, and can be turned on for individual windows,
or (with for_window) for all new windows. See the userguide change.

This commit is partially based on work by:

• Marius Muja
• mickael9
• Esteve Varela Colominas
• Bernardo Menicagli

include/data.h

@@ -15,6 +15,7 @@
 #include <xcb/randr.h>
 #include <pcre.h>
 #include <sys/time.h>
+#include <cairo/cairo.h>
 
 #include "queue.h"
 
@@ -471,6 +472,9 @@ struct Window {
     double min_aspect_ratio;
     double max_aspect_ratio;
 
+    /** Window icon, as Cairo surface */
+    cairo_surface_t *icon;
+
     /** The window has a nonrectangular shape. */
     bool shaped;
     /** The window has a nonrectangular input shape. */
@@ -656,6 +660,11 @@ struct Con {
     /** The format with which the window's name should be displayed. */
     char *title_format;
 
+    /** Whether the window icon should be displayed, and with what padding. -1
+      * means display no window icon (default behavior), 0 means display without
+      * any padding, 1 means display with 1 pixel of padding and so on. */
+    int window_icon_padding;
+
     /* a sticky-group is an identifier which bundles several containers to a
      * group. The contents are shared between all of them, that is they are
      * displayed on whichever of the containers is currently visible */