inline if didn't work. Added intermediate register_event function in transition library

lib/darktable_transition.lua

@@ -0,0 +1,51 @@
+--[[
+
+    darktable_transition.lua - temporary library to help with API transition
+
+    Copyright (C) 2016 Bill Ferguson <wpferguson@gmail.com>.
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+]]
+--[[
+    darktable_transition - routines to maintain compatibility with new and previous API versions
+
+]]
+
+local dt
+for k,v in pairs(package.loaded) do 
+  if string.match(k, "darktable") then
+    dt = v
+    break
+  end
+end
+
+if dt then
+  dt.orig_register_event = dt.register_event
+
+  function dt.register_event(name, event, callback, tooltip)
+    if dt.configuration.api_version_string >= "6.2.1" then
+      if tooltip then
+        dt.orig_register_event(name, event, callback, tooltip)
+      else
+        dt.orig_register_event(name, event, callback)
+      end
+    else
+      if tooltip then
+        dt.orig_register_event(event, callback, tooltip)
+      else
+        dt.orig_register_event(event, callback)
+      end
+    end
+  end
+end