Add room objects.

(Move from .ctl into Graphics.ref).
Add exterior map.

(Move from .ctl into Graphics.ref).
Add super tiles to map page.
Mark text strings with 't' rather than 'b'.

This means that the .skool file is generated with DEFB and DEFM
directives which look odd as the text is not ASCII, but at least the
messages appear in their own section of the disassembly now.
Makefile: Add a 'clean' rule.
Turn incorrect 'REFS' directives into 'R'.