DBBrowserDB: Always update internal variables after changing the DB layout

Make sure to always replace the old DB layout representation in
DBBrowserDB after making changes to the database.
EditTableDialog: Make it possible to change the type of existing columns

Restore the possibility to change the data type of a column of an
existing table - a feature we lost some time ago.

Also change the updateTypes() method to not actually update the types of
all columns when only one can be changed at a time.