Add support for VariableSubstitutions.txt file

[ghorwin]

File contains a table with two columns (tab separated), first holds original quantity name, second holds substituted quantity name.

The file is read and added to a session (as session property). The list can be edited manually (i.e. via push-button in property widget a list definition opens, where the substitutions can be entered/read from file/stored in file).

The substitutions are used session-wide.

Whenever the user interface retrieves the extractors quantity names, these are first run through the substitution list. Hence, all access to extractor quantities should be piped through some global/session-wide substitution function.