Add GTK4 deprecation status report with migration roadmap (#9)

* Initial plan

* Initial plan for GTK4 deprecation analysis

Co-authored-by: laeubi <[email protected]>

* Add comprehensive GTK4 deprecation status documentation

- Analyzed 159 deprecation warnings from build_gtk.sh output
- Categorized 149 unique deprecated functions into 23 categories
- Created detailed migration plan with 4 phases (simple to complex)
- Documented automation potential for AI-assisted migration
- Added GTK4 migration guide references and resource links

Co-authored-by: laeubi <[email protected]>

* Delete build_gtk.sh

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: laeubi <[email protected]>
Co-authored-by: Christoph Läubrich <[email protected]>

Author: Copilot