Script to update new features of plt_def.pro

[ericg]

The goal is to keep the user's settings while adding new features in plt_def.pro (or other config files). Method proposed: a script that scans the new plt_def.pro for new features and add them to the user's plt_def.pro (at the right place...). May use svn fonctions to look at the differences and then decide if the difference is about a new variables and not a different setting of an old variable