Versioning attribute in project files

[herman@…]

Some enhancements and bugfixes will change Cinelerra's behaviour in a way that is incompatible. That is, old projects will no longer behave or render as they used to, meaning a new version of Cinelerra can no longer load and handle certain old projects correctly.

To deal with this, we need a version attribute in the project files, and extra code to retain the "original" behaviour in the old version of Cinelerra that created an older project file. This will take some care and thought...