Sets and returns the handler name for the command as a String.
The handler specifies the name of the function or subroutine that implements
the command. In other words, the Handler is the name of the callback function
that Softimage calls each time the command is invoked.
Note: It is possible to implement a simple script-based command without a handler.
Such a command cannot have any arguments and Softimage will execute the code at the
global scope of script rather than calling a subroutine.
For commands implemented with the C++ API this property is not necessary
because the handler is always OnXSICommandCPP(). For more information see the
Custom Commands chapter of the SDK Customization guide.