[Lazarus] Extending FCL documentation

[Hans-Peter Diettrich]

In the meantime I added another feature to the FPDocManager, that shall 
allow to add description directories to documentation packages. The GUI 
is a bit rudimentary for now, deserves some improvements.

This feature can e.g. be used to add the descriptions in 
$Lazarus/docs/xml/fcl to the official FCL documentation. At least this 
part works already :-)

DoDi