eZDBInterface: Constructor and class var 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. <del>Changed the access of the class variables to what has beed defined in the `@access` PHPDoc properties</del>
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