Check / Improve the handling of the modified state in the base entity

In this Issue https://jira.translate5.net/browse/TRANSLATE-3464 the modified-state tracking was improved in the base entity class. Still, it is not correct as loading a new entity (row) does not reset the tracking of modified fields/values. Unfortunately when adding this, a lot of API-tests fail. These must be investigated / recaptured. Another improvement would be, to reject setting fields/columns with the identical value there already is. In this case, the modification should not be captured. Worth respecting is how to deal with differing types, e.g. saving an int to an int-field (but the db-value is string). This would avoid unneccessary db-operations in some cases.