Add callback to all methods changing node data

[lukaszachy]

Similar to #197 but for other fmf features as inheritance and merging.

Goal is to make caller aware of such changes and let it report the the user to find out e.g. invalid merges

[lukaszachy]

I'd say it callback should receive: node name, old value, new value, link to the line in source file (if exists)

edit: how about operation (set, + +< -) and key? Also /: inherit:false should be reported somehow.

[fernflower]

Thank you, having lost some hours to debugging we'd really appreciate this!