awful.titlebar: Unreference titlebars in unmanage

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

lib/awful/titlebar.lua

@@ -296,6 +296,10 @@ function titlebar.widget.stickybutton(c)
     return widget
 end
 
+client.connect_signal("unmanage", function(c)
+    all_titlebars[c] = nil
+end)
+
 return setmetatable(titlebar, { __call = function(_, ...) return new(...) end})
 
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