Something is missing. Consulting the manual, I have found out that I need LibFinder for "Project -> Properties -> Libraries". I do not seem to have LibFinder installed. "Managing Plugins" wants a plugin to install. Therefore,
(1) Where do I get LibFinder? How do I install it?
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Okay, install "wxsmithlib0" and "contrib" packages. You will get LibFinder. Now, you have "Project -> Properties -> Libraries" with gtkmm. Also set "Target - GUI" and everything will work. But I am a lazy tom-cat, isn't it possible to automate the gtkmm settings? Also, when I quitted from CodeBlocks, I got a message that I have changed defaults and whether I am about to save it. Not all my programming is planned to be gtkmm so that I wouldn't want to have gtkmm settings as a default. I saved the changes in default, did I saved gtkmm as a default?