Tests: Add unit test for CHECK NOT LIKE constraint

Add a unit test for the grammar fix in
9cb0614ecfad1536c18e17061fa3aa48b26c1f68.
Tests: Make sure all unit tests pass

Two unit tests where failing because of missing spaces in the expected
result string.
MainWindow: Make it possible to cancel closing of database file

When closing the database, either by using the menu item or by closing
the window, the user is asked whether the changes he made shall be
saved or not. Add a third button to this message box which makes it
possible to cancel the action.