Rollback introduction of +get and +set folders

Under Octave, having namespaces called “get” and “set” overshadows the builtin
functions with the same names, which are needed for graphics manipulation.

Therefore we go back to the initial function naming scheme, but moving all
those functions under an “accessors” subdirectory.

Among other things, this is a revert of
e4134ab5 and
c5e86fcb.

Ref. !1655, !1686