eZDBInterface: Small refactoring

Greetings!

I have tried to be as atomic as possible with the commits to make it easier to follow the changes. Here is a summary:

1. Moved the class variables from the bottom of the class to the top
2. Added class variables which have previously been set dynamically
3. Changed the access of the class variables to what has beed defined in the `@access` PHPDoc properties
4. Default values are now set directly in the class variables instead of in the constructor
5. Added checks in the constructor, if a key in the `$parameters` array is actually present before using it

If (2) is too intrusive to make you accept this PR, I would like to revert the changes instead of rejecting the PR.

Cheers
:octocat: Jérôme