Always initialize preference defaults using the extension point

[torokati44]

This aims to solve https://www.brainwy.com/tracker/PyDev/956.

The problem was that calling the initializeDefaultPreferences() methods manually (directly) caused the defaults loaded from plugin_customization.ini to be overridden. According to the documentation, this should never be done, and using the extension point is the (only) proper way.

Now, there were comments I deleted, saying something about backward compatibility. What exactly are those about? Are they still relevant today?

I also renamed some constants that had confusing names, just for consistency's sake.

[fabioz]

I'm sorry about the long delay in getting back to you on this issue... are you still interested in solving the conflicts to have that change incorporated?

[torokati44]

I think we were able to work around this problem somehow since, so it's no longer that urgent for us, but doing things properly is always nicer IMHO.
Having this merged would perhaps make our code simpler too, but it's not a priority anymore. So, it's up to you...