BUG: Ensure backward compatibility in qMRMLNodeComboBox

Here is a quick summary of what happened:

What changed in r22052:
 Rename the Q_Property from currentNodeId to currentNodeID
 Rename QString currentNodeId() to QString currentNodeID()
 Rename setCurrentNode(const QString nodeID)
  to setCurrentNodeID(const QString nodeID)

What changed in r22059:
 Add QString currentNodeId() as a deprecated function
  (not invokable though)

What's breaking:
 In C++, the function setCurrentNode(const QString nodeID) is missing.
 In Python:
  The function currentNodeId() is missing (not wrapped due to a missing
   Q_INVOKABLE).
  Trickier, any code using the property currentNodeId = ... will break too.

Proposed fix:
  Add Q_INVOKABLE to currentNodeId() method for python wrapping.
  Return the old property as deprecated.

See Issue #1694