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

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