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

sway/commands/bar/tray_output.c

@@ -1,7 +1,29 @@
+#define _XOPEN_SOURCE 500
+#include <string.h>
 #include "sway/commands.h"
-#include "log.h"
 
 struct cmd_results *bar_cmd_tray_output(int argc, char **argv) {
-	sway_log(L_ERROR, "Warning: tray_output is not supported on wayland");
+	const char *cmd_name = "tray_output";
+#ifndef ENABLE_TRAY
+	return cmd_results_new(CMD_INVALID, cmd_name, "Invalid %s command "
+			"%s called, but sway was compiled without tray support",
+			cmd_name, cmd_name);
+#else
+	struct cmd_results *error = NULL;
+	if ((error = checkarg(argc, cmd_name, EXPECTED_EQUAL_TO, 1))) {
+		return error;
+	}
+
+	if (!config->current_bar) {
+		return cmd_results_new(CMD_FAILURE, cmd_name, "No bar defined.");
+	}
+
+	if (strcmp(argv[0], "all") == 0) {
+		// Default behaviour
+		return cmd_results_new(CMD_SUCCESS, NULL, NULL);
+	}
+	config->current_bar->tray_output = strdup(argv[0]);
+
 	return cmd_results_new(CMD_SUCCESS, NULL, NULL);
+#endif
 }