Separate out draw functions

[bolt-blue]

Description

Draw functions don't really belong in screen.c.
Also, as and when more are added, it would further exacerbate the above.

• The separation should not mean multiple includes for user code
  • a single termadore.h has been suggested, that includes all submodules
• drawutils.c is the proposed separation
  • this would itself also rely on screen.c

Questions

• How to give drawutils access to the screen object, without making it public to user code?
  • or should the same facility be provided to user code?

Proposals

• screen.c provides access to its screen struct.
• drawutils.c makes use of the above - users may or may not at their will
• Compile screen and drawutils into a single shared object, or not?

[awth13]

The current Makefile implementation for the static libtermadore.a library fails to compile on demand:

make: *** No rule to make target '../src/libtermadore.a', needed by 'colours'.  Stop.