Provide unified way of handling properties references for connections, datastores, and datasets

Connections

1. In DI, BD connection properties could be both: embedded or referenced (from existing component list - either a connection or configuration component)
2. In TFD they are always referenced (from external datastore list)

Datasets

1. In DI, BD they are embedded
2. In TFD they are always referenced

See https://wiki.talend.com/display/rd/Significant+Design+issues+for+TCOMP+1.0 for details

Things to change:

1) Component Reference mechanism in the studio uses a COMPONENT_REFERENCE widget type. This, in the studio, because target execution environment is DI, give a choice of an embedded component properties, or a reference to another component's properties. If embedded, this widget will create the properties, and render the MAIN form to configure these properties right in line below the reference type (this, <list of components>) combobox. If it's not embedded, the combo box shows the referenced component.

This changes the way the SalesforceConnectionProperties is handled, the referencedConnection in the SFCP is removed, and the component reference just points to the desired SFCP.

2) The design environment can change the way references are handled based on the class (or superclass) of the target of the reference. For example, when designing for DI, the references whose target is a DatastoreProperties can be any component in the job whose main property class is DatastoreProperties (this is the top-level properties object pointed to by the component). Or it can be embedded. But for TFD, the reference will always be embedded as the properties are somehow injected into the component. The reference widget will not even appear in the TFD case. For datasets, it's different, as even in DI, it is embedded.