In this course, more code which was in SVTXGridControl was moved to the places it belongs to, effectively
meaning that the TableControl has better chances than before to be used outside an SVTXGridControl (though
we're not there, yet).
Also, the selection-related methods in TableControl/_Impl got some refactoring love, so the implementation
details are hidden in the Impl class now, instead of being exposed to an exploited by the TableControl.
And while we were there ... The XGridSelection does not provide a |selectColumn| anymore (there was no
preparation whatsoever for column selection, anywhere, thus it was a complete dummy.)
Also, its de/selectRows methods have been removed: They have questionable use, and make implementation
rather difficult, compared with multiple calls to de/selectRow.
