Add a class TOOL_MENU, which provides a management class for a
CONDITIONAL_MENU and a set of CONTEXT_MENUs. The aim of this is to
provide a central place where all TOOL_INTERACTIVEs can get a "basic"
context menu that either they or other tools can register new items and
sub-menus against.
This means that "top-level" tools no longer need to manage the lifetimes
of any CONTEXT_MENUs that they add, and can also delegate simple menu
display functions.
John Beard