enabling write access to list of installed converters. Refs: #122.
As proposed by Doug Danforth.
This is an experimental trivial solution to the requirement of removing an installed converter by a program,
for example when unloading the implementing module.
The anchor of the list is still read-only and contains the .odc converter.
The solution may be changed to a more structured form in BlackBox 1.8
after experience has been gathered with this approach.

Signed-off-by: Josef Templ <josef.templ@gmail.com>