Changes on ExternalSource, from 1.8 to 1.12
(1.10 to 1.14 for VisualWorks 2.0)

bug fixes:
	- mnu #nextChunkPut: during file-out of some OtherChange!
	- improper source/changes file reference after #invalidSourceFileAction:with:
		(source file access failure imposes discarding the file entry 
		without Change objects (used as persistent source code pointers 
		in Application Management) becoming aware; 
		redefining a proper value did not update persistent Change objects; 
		to overcome dependency on system source file entries the inst var 
		'file' may store Integer values referring to the source file of the 
		SourceFileManager respectively

modifications:
	source file access error handling has been improved
	- the invalid file entry will no more be discarded (set to nil)
	- smarter reaction when running out of file handles
	- improved error messages, displaying the reason for the 
		source access failure, trying to find out which CompiledMethod 
		holds the invalid source pointer 

NOTE:
	- in order to save consistency of current change objects,
		this upgrade tries to map existing file references to appropriate 
		integer values. If you ever lost your source/changes file entry
		due to invalid source file handling of VisualWorks then it may
		happen, that change object refer to some file which does not 
		exist anymore. The consistency check will then request what 
		to do. The question displayed is 

			'One of the changes refers to file ...
					Tell me how to map it'

		with '...' replaced by the name of the file referred to. 
		You will be given the following options:

			- sources
				converts the file entry of the change into a reference 
				to the image source file

			- changes
				converts the file entry of the change into a reference 
				to the image changes file

 			- debug
				interrupts the conversion and opens a debugger
				on the current execution context.
				Do not forget to proceed the process, otherwise
				following changes of the upgrade process may be lost.