Implement auto filter sheet storage.

This is for auto filter stored directly in the sheet only.
Test the import of sheet auto filter data for real.
Move this from a public header directory.

We don't need to expose the implementation of an interface.
Update slickedit project files.
Add class descriptions.
Move table.hpp from public directory to private one.
Note about ixion dependency in public header files.
Initial data structure set for storing table data.

I put them in the same place with the autofilter data, since the two
are quite similar in nature and usage.
Work on picking up table data.  Not done yet.
Parse tableColumn elements in the table part.
Parse tableStyleInfo element inside table.
Pass the table interface as reference, not as pointer.

The pointer has already been checked for NULL-ness earlier in
read_table() call.
Common method to push auto filter data to the model via interface.
Pass picked-up table data to the table interface.
Calling get_auto_filter() should reset its internal state.

The same with any other getter methods that return a feature interface.
Let's make this a rule.  With this rule, the filter code needs to call
the getter at the beginning only once since calling it will initialize
the state of its internal feature data.
Add note about spreadsheet's types.hpp header.
Store table data to the document model for real.
Start writing test for table import.

Some parts still fail.