* Theory loader: be more serious about observing the static theory header specifications (including optional directories), but not the accidental file locations of previously successful loads. Potential INCOMPATIBILITY, may need to refine theory headers.
* Theory loader: optional support for content-based file identification, instead of the traditional scheme of full physical path plus date stamp; configured by the ISABELLE_FILE_IDENT setting, (cf. the system manual). The new scheme allows to work with non-finished theories in persistent session images, such that source files may be moved later on without requiring reloads.
