Add PropertyHandler and used it at some points to clarify the usage.

Introduced PropertyHandler to hold the key and the default value of a property. They're responsible to read and transform the value - and vice versa to write it back.
The advantages:
* The compiler checks the types. It's impossible to store a wrong type into a property
* Enums are translated back in one line
* Default values are configured at one place
* Listeners realized with PropertyChangeSupport of java.beans

I used it for most of the connection properties. But feel free to use them for other properties too.