Create savepoint when editing a table

Create a savepoint when starting to edit an existing table, release it
when the user clicks on the OK button and rollback to it if he cancels
the action.
Extend remove table column function

Make it possible to remove columns in the dialog shown to design a new
table.
Show a TODO message when user tries to delete a column in an existing
table.
Show a TODO message when trying to edit column of an existing table

Don't allow the user to edit the columns of an existing table and show a
warning instead. It's not working anyway.
Show those TODO message also when using the toolbar or the popup menu

This should let the user know that something's not working instead of
offering him a not working dialog and winding him up this way. That
should make the behaviour of the program much easier to understand.