Update slickedit file.
Use string_id_t to store string identifiers in all places.
Hide formula_functions.hpp header.
Hide formula_interpreter.hpp too.
Update slickedit project file.
Remove unused header includes.
Hide depends_tracker.hpp.  This is also an implementation detail.
Use pimpl for cell_listener_tracker.
Just use boost::unordered_set.
Hide this private method.
Move ixion_test into src/libixion.
Move session handler implementation out of libixion.

It makes more sense to move it to ixion-parser than to leave it in
libixion.
Rename table interface to table_handler interface.

To be consistent with session_handler...
Set up skeleton table_handler implementation.
model_context impl to NOT manage the life cycles of the handlers.

Let the client code manage it.
Add support for defining tables. Still work in progress.

I just need to work on parsing the column definitions.
Parse table column names.
Add dummy SUBTOTAL function. For now it always returns 123.