Use 32-bit visuals for i3bar when possible and allow RGBA colors.

This patch creates all necessary windows for i3bar with 32-bit visuals if available.
It also introduces the possibility to define RGBA colors (next to RGB colors), which
allows the user to set the opacity of any color. This requires running a compositor.

With this patch we also start supporting _NET_SYSTEM_TRAY_VISUAL, which is necessary
for the tray icons so they create the tray window with the correct depth and visual.

i3bar/include/cairo_util.h

@@ -15,6 +15,7 @@ typedef struct color_t {
     double red;
     double green;
     double blue;
+    double alpha;
 
     /* For compatibility, we also store the colorpixel for now. */
     uint32_t colorpixel;