Table skin classes: can't extend due to abstract package private methods

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.

Actually, we can't even extend the class - compiler error:

"This class must implement the inherited abstract method TableCellSkinBase<TableCell<S,T>>.columnWidthProperty(), but cannot override it since it is not
visible from XTableCellSkin. Either make the type abstract or make the inherited method visible"