Unify the hex fallbacks for unknown DWARF constants
Start specializing data attributes, starting with language
Display DW_AT_decl_file too

This requires keeping a Dwarf_Die copy along with each attribute row, so
Dwarf_Files can be reached, but the attrtree is usually small anyway.
configure: make sure elfutils is 0.148+
Require glib 2.32 for glib-compile-resources

Might as well claim gtk+ 3.4, since that came out at the same time.

Also, add the matching MIN_REQUIRED / MAX_ALLOWED, so the project can be
kept clean to those requirements.