Allow setting the default value, the check values and under some
circumstances the not null flag when editing existing tables.
This required some changes to DBBrowserDB::renameColumn() which is now
using some more features of sqlitetypes.cpp but could still be improved.
Do just the same in the ImportCsvDialog as before in the
EditTableDialog, i.e. using a unique savepoint name which is not
released when the import was successfull.
Rewrite the dialog to import CSV files using a Qt Creator form file.
Simplify the CSV import a little bit.
A bit unrelated, but needed for this anyway:
Create the restorepoint right after opening a database file instead of
just doing so after the first change.
