Allow NULL for the callback in cpdbGetNewFrontendObj()

[prab0007al]

@tillkamppeter

Description

This pull request enhances the cpdbGetNewFrontendObj() function to accept NULL for the callback parameter. When NULL is passed, the function will use the default cpdbPrinterCallback() function, which logs printer updates to the console. This makes the API simpler to use for applications that don't need custom printer update handling.

Changes

• Modified cpdbGetNewFrontendObj() in cpdb-frontend.c to use the default callback when NULL is passed
• Added a declaration for cpdbPrinterCallback() in cpdb-frontend.h to make it accessible to users
• Updated documentation in cpdb-frontend.h to reflect this change
• Updated the example in cpdb-text-frontend.c to demonstrate using NULL for the callback
• Updated CHANGES.md to document this enhancement

Fixes #80 - Allow NULL for the callback in cpdbGetNewFrontendObj()