Put all loaded layers in a separate group
[VERSION 0.2.2] commit all changes immediately on OK

Load n2m tables as layers and edit within QGIS

Set related layers editable on opening and commit/rollBack on closing
the mask.
Fix error in accessing feature value in Ddn2mTable
Set Db for layer on initializing (optional)
Register change if only notNull checkbox has been clicked
Use Python int instead of QLong in QInt64Validator
Deactivate checkbox for NotNull TextEdits with ...

...no text (happens with new features)
Connect slots in the new way