libgimpbase, app: add variants of gimp_$foo_directory() which return GFiles

gimp_directory_file(), gimp_data_directory_file() etc. The new
functions take a variable list of path elements to the file,
the list has to be NULL-terminated. Remove the newly added
gimp_personal_rc_gfile(). Start using the new functions in app/.
app: use gimp_directory_file() in session.c