Add possibility to write properties-files

[FelixSFD]

Right now, the properties are read-only. It should be possible (and easy) to save changes.

• add interface EnhancedWriteableProperties which extends EnhancedProperties with:
  • default setters (int, double, String,...)
  • save() method
• add abstract class EnhancedWriteablePropertiesImpl which implements the new interface and extends EnhancedPropertiesImpl
• make EnhancedPropertiesInFile and EnhancedPropertiesInFileOrResources extend the new class
• add event listeners to listen for changed properties