VERIFIED SOLUTION i

How to configure the "External Editor" option for a message type defined in Dialogue Admin - Portrait Dialogue

In Dialogue Admin's Message Type Properties there is an Editors & viewers tab where it is possible to define a Master template editor and a Template editor. When the External Editor option is selected from the drop down list for message and master templates, like this...
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...message and master templates will be opened in an external editor.
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To make it work with an editor of your choice, set the default editor for the appropriate file extension (.txt type file in Notepad ++ for example).
This can be done using the Windows Registry Editor and setting the relevant values in the HKCR\SystemFileAssociations\text\shell\edit\command path.
[Please note that you should never manually change the Windows Registry unless you know what you are doing. This is done at your own risk.]
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An alternative way to safely achieve the same thing without manually editing the Windows registry, is to use a 3rd party tool such as Default Programs Editor.
 
UPDATED:  April 19, 2017