Add progress notification via callbacks (a la Event Listeners)

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Problem
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* Currently, Thumbnailator does not have facilities to provide feedback as to 
the progress of a thumbnail generation task.

This means that if a task takes a long time, there is no way for the 
application using Thumbnailator to give feedback as to the progress of making 
thumbnails.

Also, not providing adequate feedback means that applications using the library 
may not be able to perform adequate logging, which may result in difficulty 
during development and debugging.

Solution
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* Provide an event notification facility, similar to the `EventListener`s which 
can be found in the Java APIs.

Complications
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* Many classes would have to be retrofitted to support the event notification 
mechanism.

Original issue reported on code.google.com by coobird...@gmail.com on 3 Apr 2011 at 11:32

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Original comment by coobird...@gmail.com on 26 Feb 2012 at 6:23

• Added labels: Priority-Low
• Removed labels: Priority-Medium

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Original comment by coobird...@gmail.com on 22 Dec 2013 at 7:15

• Changed title: Add progress notification via callbacks (a la Event Listeners)