[TxMt] XCode - Clickable error messages

[Chris Thomas]

«file»:«line»(:«column»)?:«error>> sequences are now clickable, modulo  
any lurking bugs. Underpinning this feature is new reusable  
infrastructure:

• Add raw HTML markup (to be interpreted as HTML) to the output.
• Given only the basename, find the full path of the source file in  
the Xcode project. This being the difficult part.

It should now be relatively easy to add different link formats as  
needed (and to support HTML output).

Chris