Allow multiple tray_output directives.

This patch introduces the possibility to specify the tray_output directive
multiple times. All values will be used by i3bar, in the order they are
given.

This way, a single bar configuration can be used for several machines with
internal output names "eDP1" and "LVDS-0" by specifying tray_output for both.
Any external output (e.g., "DP-0") will still not receive the tray. The same
effect can be achieved by using "primary", but forces the user to couple the
tray display to the primary output which may not be desirable behavior.

relates to #555

docs/userguide

@@ -1369,6 +1369,11 @@ NetworkManager, VLC, Pidgin, etc. can place little icons.
 You can configure on which output (monitor) the icons should be displayed or
 you can turn off the functionality entirely.
 
+You can use mutliple +tray_output+ directives in your config to specify a list
+of outputs on which you want the tray to appear. The first available output in
+that list as defined by the order of the directives will be used for the tray
+output.
+
 *Syntax*:
 ---------------------------------
 tray_output none|primary|<output>