[dbmanager] Fix non-clickable checkbox in Null column when creating table (on Mac)

- Switch from checkbox delegate to Qt::CheckStateRole for column
- Enable all triggers for table view
[dbmanager] Fix settings default and Python type for connection with postgis plugin

- Saved connections with empty password raised a QPyNullVariant parameter error due to lack of str default
[dbmanager] Add missing InvalidDataException import
Switch targets staged_[plugin_dir_name] to staged-[plugin_dir_name], as noted in comments

- Only plugin directories should have underscores, targets should have hyphens