Convert *Base (e.g. PropertiesBase, HelperBase) classes to XTend2 classes

[rcodesmith]

The *Base classes (e.g. PropertiesBase, HelperBase) are used in transformations and templates, but are implemented in Java, so cannot be overridden via the Sculptor extension mechanism.

These classes were in Java prior to the XTend1/XPand -> XTend2 migration, so have remained as Java classes.

This issue is to track the conversion of these methods into XTend. These methods can usually be placed in the corresponding non-Base XTend2 class (e.g. Properties, Helper, DbHelper).

[tjuerge]

This approach "tackling one-by-one and marking the migrated (but still needed) ones as deprecated" is very good.