Python plug-ins and some general principles

Dear all (in particular Darcy),

One developer reported a problem concerning the Python-based plug-ins under Windows
(maybe also on other platforms): if Python is not installed, then this provokes
dozens of error messages.  Could you please clean this up?

As a general rule, plug-in initializations should be extremely minimalistic and
error-proof.  You mainly should just determine what kind of functionality is
present and do not do anything else.  Modules should be loaded only when
the plug-in is actually started.  This is both for safety and efficiency.

Best wishes, --Joris