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

CMake/FindDBus.cmake

@@ -0,0 +1,59 @@
+# - Try to find DBus
+# Once done, this will define
+#
+#  DBUS_FOUND - system has DBus
+#  DBUS_INCLUDE_DIRS - the DBus include directories
+#  DBUS_LIBRARIES - link these to use DBus
+#
+# Copyright (C) 2012 Raphael Kubo da Costa <rakuco@webkit.org>
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+# 1.  Redistributions of source code must retain the above copyright
+#     notice, this list of conditions and the following disclaimer.
+# 2.  Redistributions in binary form must reproduce the above copyright
+#     notice, this list of conditions and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER AND ITS CONTRIBUTORS ``AS
+# IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+# PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR ITS
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+# EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+# PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
+# OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+# WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
+# OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+# ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+FIND_PACKAGE(PkgConfig)
+PKG_CHECK_MODULES(PC_DBUS QUIET dbus-1)
+
+FIND_LIBRARY(DBUS_LIBRARIES
+    NAMES dbus-1
+    HINTS ${PC_DBUS_LIBDIR}
+          ${PC_DBUS_LIBRARY_DIRS}
+)
+
+FIND_PATH(DBUS_INCLUDE_DIR
+    NAMES dbus/dbus.h
+    HINTS ${PC_DBUS_INCLUDEDIR}
+          ${PC_DBUS_INCLUDE_DIRS}
+)
+
+GET_FILENAME_COMPONENT(_DBUS_LIBRARY_DIR ${DBUS_LIBRARIES} PATH)
+FIND_PATH(DBUS_ARCH_INCLUDE_DIR
+    NAMES dbus/dbus-arch-deps.h
+    HINTS ${PC_DBUS_INCLUDEDIR}
+          ${PC_DBUS_INCLUDE_DIRS}
+          ${_DBUS_LIBRARY_DIR}
+          ${DBUS_INCLUDE_DIR}
+    PATH_SUFFIXES include
+)
+
+SET(DBUS_INCLUDE_DIRS ${DBUS_INCLUDE_DIR} ${DBUS_ARCH_INCLUDE_DIR})
+
+INCLUDE(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(DBUS REQUIRED_VARS DBUS_INCLUDE_DIRS DBUS_LIBRARIES)