systray: Turn this into a singleton

There's no point in having multiple instances of this, because there are no
per-instance settings and the systray can only be visible in a single place at a
time anyway.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/widget/systray.lua

@@ -90,8 +90,12 @@ local function new(revers)
     return ret
 end
 
+local instance
 function systray.mt:__call(...)
-    return new(...)
+    if not instance then
+        instance = new(...)
+    end
+    return instance
 end
 
 return setmetatable(systray, systray.mt)