When your IDE is running as an AppImage, the $APPRUN variable is
already set in the built-in terminal. Now of course you could just
calculate the path of $APPRUN yourself and force-overwrite it in that
script; however that's more effort and more likely to break.
Therefore it's easier to just introduce and use a custom variable.
Makes adding more features easier, as there's a clear defined interface
to the class and the mechanism can be split into more methods more
easily without bloating the AppDir class further.
