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

swaybar/config.c

@@ -48,6 +48,18 @@ struct config *init_config() {
 	/* height */
 	config->height = 0;
 
+#ifdef ENABLE_TRAY
+	config->tray_output = NULL;
+	config->icon_theme = NULL;
+	config->tray_padding = 2;
+	/**
+	 * These constants are used by wayland and are defined in
+	 * linux/input-event-codes.h
+	 */
+	config->activate_button = 0x110; /* BTN_LEFT */
+	config->context_button = 0x111; /* BTN_RIGHT */
+#endif
+
 	/* colors */
 	config->colors.background = 0x000000FF;
 	config->colors.statusline = 0xFFFFFFFF;