Example extensions are hard to understand

[petebankhead]

At some point the example extensions (especially the Java one) have become complicated and confusing.

Some issues:

• The fxml UI is too small by default
• The spinner changes 'threads' but it's not clear to the user if this will actually impact the functioning of the software
• The run button doesn't seem to do anything
• The use of string resources is inconsistent, so doesn't show best practices
• There is no parent or title set for the dialog that is shown, so it doesn't behave like most in QuPath
• Uses deprecated API (addPropertyItem)
• addPreference and addPreferenceToPane do the same thing
• More fields could be final

I think the demo should be either very minimal (e.g. show a dialog and nothing more)l, or show very clear examples of best practices (e.g. show the use of fxml and resources).