Fix order of arguments for onContentBeforeSave event

[parapente]

Fixes #171

[JLTRY]

Seems good to me https://manual.joomla.org/docs/5.4/building-extensions/plugins/plugin-events/content/#oncontentbeforesave
I can not reproduce as none of my plugins use this event

[parapente]

Note that both item and data show the data which is going to be saved, in other words, the modified data. Neither shows the data values which are currently in the database.

Hmmm... Maybe I should not add null as the $data argument and use $model as its value. Unfortunately I don't have any plugin that will use this event and the $data argument. This event was added to achieve indexing of the attachments. The finder plugin uses only the first two arguments of the event. I will update the PR later this day.

[parapente]

For some reason moving the event triggering breaks attachments on newly created articles. This PR is not ready for merge yet.

[parapente]

This should work just fine now. Please test

[JLTRY]

I propose to merge