Add support for item selection to JFreeChart plots

Added support for item selection in JFreeChart plots.
Examples can be found in demo/selection.

- ChartEntities are used to compare selected screen areas against rendered objects
- DatasetExtensions can extend existing datasets with additional information e.g. selection state
- Item Rendering Strategies allow a more fine granular rendering of items e.g. dependent of their selection state.

The first commit in the selection_clear_history branch sums up the changes against
the current master to get things working. The subsequent commits focus on porting the
added code to a more java 1.6 like coding style. The original implementation which was
based on JFreeChart 1.0.14 can be found here https://github.com/MichaelZinsmaier/JFreeChartSelection.

Because I implemented the code originally for 1.0.14 I'm not sure if it might overwrite changes that have been introduced by the previous work on the FSE. E.g. my code changes some things about the MouseHandlers and if they have been improved since version 1.0.14 I might accidently overwrite these changes by porting the selection code to the FSE. However I am not aware of such issuese at the moment (-:

Greetings from Konstanz
Michael