Implement and document pgfkeys tracing.

[sasozivanovic]

Motivation for this change

In 2012, Ryan Reich wrote a handy little utility for tracing the execution of pgfkeys processing. However, his unpublished trace-pgfkeys.sty had a form of a runtime patch and stopped working upon changes in pgfkeys. In 2016, I took over maintainance of the package, by agreement with Ryan, with the idea of integrating the tracing code into pgfkeys itself. Here it is, finally. The DocStrip-like approach makes it easy to maintain, while incurring no overhead when tracing is not in effect.

While the framework integrating the tracing code into pgfkeys is of my implementation, the tracing messages (and trace levels) are mostly Ryan's, with a couple of my additions. Only the core pgfkeys.code.tex is equipped with tracing code; I'm not familiar enough with key filtering to add tracing messages to pgfkeyslibraryfiltered.code.tex.

[hmenke]

https://sourceforge.net/p/pgf/patches/23/ was never migrated it seems.

[sasozivanovic]

https://sourceforge.net/p/pgf/patches/23/ was never migrated it seems.

That's ok. That implementation was too complicated.