Port to SIP 4.19

We are currently stucked with 4.18. This is getting more and more an issue since LImA is "incompatible" with every packages that have sip 4.19 as dependency (i.e. pyQt5).

There are a number of incompatbilities between 4.18 and 4.19 that need to be addressed.

This one is especially annoying:

As described in Generated Type Structures SIP generates a type structure for each C structure, C++ class and namespace, mapped type and named enum. Prior to this version every type structure in a module was automatically available to any handwritten code in any module that imported it.

And there is also an issue with the code generated for exceptions.