Add project_write_config() to force project file rewrite

As I mentioned, I'd like to add a feature to my gproject plugin to add external directories which can be accessed from the project sidebar and which are indexed by the tag manager (done already in my private copy). The problem is that right now, the only way to write the project file is through the project-save signal handler which is only emitted from the project properties dialog. However, I'd like to be able to add the external directory from the sidebar (project dialog isn't really the right place for this) and right now there's no way to force the resubmission of the project-save signal - and this is what this new API call does.