undo: generate fieldId() virtual functions by templates

Most edit commands derive from a common base class EditBase,
which declares a fieldId() virtual function that has to be
defined by the child classes. This is tedious. For some reason
the C++ makers refuse to allow "virtual member constants".

To make the code somewhat less verbose, create these functions
by a template. Of course, we could introduce the template
parameter directly in the EditBase class. However, that would
mean that the code in this base class is generated for every
single undo command. I'm not sure we want that.

This should als make it somewhat less tedious to create new
edit commands by copy & paste.

We could do the same for the fieldName. However, that is more
complicated for two reasons:

1) For historic reasons(?) C++ doesn't allow for string literals
   as template parameters. Therefore, arrays-of-string would have
   to be defined, which is not very nice.
2) We would have to make sure that these strings are recognized
   by Qt's translation machinery and use the QT_TRANSLATE_NOOP
   macro, which makes the whole thing even less attractive.
Maybe later.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

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