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

include/swaybar/tray/icon.h

@@ -0,0 +1,16 @@
+#ifndef _SWAYBAR_ICON_H
+#define _SWAYBAR_ICON_H
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <client/cairo.h>
+
+/**
+ * Returns the image found by `name` that is closest to `size`
+ */
+cairo_surface_t *find_icon(const char *name, int size);
+
+/* Struct used internally only */
+struct subdir;
+
+#endif /* _SWAYBAR_ICON_H */