Implement Tray Icons

This commit implements the StatusNotifierItem protocol, and enables
swaybar to show tray icons. It also uses `xembedsniproxy` in order to
communicate with xembed applications.
The tray is completely optional, and can be disabled on compile time
with the `enable-tray` option. Or on runtime with the bar config option
`tray_output none`.

Overview of changes:
In swaybar very little is changed outside the tray subfolder except
that all events are now polled in `event_loop.c`, this creates no
functional difference.

Six bar configuration options were added, these are detailed in
sway-bar(5)

The tray subfolder is where all protocol implementation takes place and
is organised as follows:

tray/sni_watcher.c:
	This file contains the StatusNotifierWatcher. It keeps track of
	items and hosts and reports when they come or go.
tray/tray.c
	This file contains the StatusNotifierHost. It keeps track of
	sway's version of the items and represents the tray itself.
tray/sni.c
	This file contains the StatusNotifierItem struct and all
	communication with individual items.
tray/icon.c
	This file implements the icon theme protocol. It allows for
	finding icons by name, rather than by pixmap.
tray/dbus.c
	This file allows for asynchronous DBus communication.

See #986 #343

wayland/cairo.c

@@ -8,6 +8,25 @@ void cairo_set_source_u32(cairo_t *cairo, uint32_t color) {
 			(color >> (0*8) & 0xFF) / 255.0);
 }
 
+cairo_surface_t *cairo_image_surface_scale(cairo_surface_t *image, int width, int height) {
+	int image_width = cairo_image_surface_get_width(image);
+	int image_height = cairo_image_surface_get_height(image);
+
+	cairo_surface_t *new =
+		cairo_image_surface_create(CAIRO_FORMAT_ARGB32, width, height);
+
+	cairo_t *cairo = cairo_create(new);
+
+	cairo_scale(cairo, (double) width / image_width, (double) height / image_height);
+
+	cairo_set_source_surface(cairo, image, 0, 0);
+	cairo_paint(cairo);
+
+	cairo_destroy(cairo);
+
+	return new;
+}
+
 #ifdef WITH_GDK_PIXBUF
 #include <gdk-pixbuf/gdk-pixbuf.h>