Properties file no longer overwritten with contributions.txt values unless something in the properties is amiss

All that this involved was to check if the present properties were valid or not, and replace all those that aren't (Eg: null or empty or not a non-integer in a field that is supposed to be for an integer, like version) with the corresponding vlue of the field from the contributions.txt file.  

Thus, (out-of-date) values from contributions.txt no longer overwrite valid values in the properties file.

This commit fixes [Issue #2572](https://github.com/processing/processing/issues/2572).