TableCellSkinBase: columnVisibility/Width should be protected

[fastegal at swingempire.de]

On the move into public, its abstract methods columnVisibility/Width/Property
changed scope from protected to package - this makes the base class  
rather useless
for extension: it's main job is sync'ing the cell's visible property to the
column's visible property and triggering and doing the cell's layout.
Both are possible only, if subclasses provide the column properties.

Would suggest to change scope back to protected. Or any special reasons for
not doing it?

Cheers
Jeanette