Using ImageHandleManager for managing multiple Image handles in Image class

[amartya4256]

This PR introduces ImageHandleManager, an inner class to Image responsible for managing the storage, creation and manipulation of ImageHandles. This class provides methods which can be used to dead with ImageHandle instances inside an image for managing it with convenience while hiding the complexity of how the ImageHandle lifecycle is managed inside itself.

All the places where zoomToImageHandle map is accessed for any purposes is replaced by the wrapper methods provided by ImageHandleManager. Each image initializes a final field imageHandleManager with the instance of ImageHandleManager. Every instance of ImageHandle created within the Image object is registered in the manager itself and the manager is responsible for cleaning the dangling resources when are not supposed to be used anymore or when a cleanup is needed. Also the manager creates handles in a thread-safe way, making sure no 2 consumers create the same handle at the same time.

This fixes vi-eclipse/Eclipse-Platform#562