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.c

@@ -221,18 +221,22 @@ static struct cmd_handler handlers[] = {
 };
 
 static struct cmd_handler bar_handlers[] = {
+	{ "activate_button", bar_cmd_activate_button },
 	{ "binding_mode_indicator", bar_cmd_binding_mode_indicator },
 	{ "bindsym", bar_cmd_bindsym },
 	{ "colors", bar_cmd_colors },
+	{ "context_button", bar_cmd_context_button },
 	{ "font", bar_cmd_font },
 	{ "height", bar_cmd_height },
 	{ "hidden_state", bar_cmd_hidden_state },
+	{ "icon_theme", bar_cmd_icon_theme },
 	{ "id", bar_cmd_id },
 	{ "mode", bar_cmd_mode },
 	{ "modifier", bar_cmd_modifier },
 	{ "output", bar_cmd_output },
 	{ "pango_markup", bar_cmd_pango_markup },
 	{ "position", bar_cmd_position },
+	{ "secondary_button", bar_cmd_secondary_button },
 	{ "separator_symbol", bar_cmd_separator_symbol },
 	{ "status_command", bar_cmd_status_command },
 	{ "strip_workspace_numbers", bar_cmd_strip_workspace_numbers },