Fix linker errors and compilation issues on Linux/Ubuntu 22.04 with gcc 13

[leroymusa]

This PR fixes compilation and linker errors that occur when building zkanji on Ubuntu 22.04 with gcc 13.

Issues Fixed

1. Linker Errors for GroupCategory Template

Problem: The build failed with undefined reference errors:

• undefined reference to 'GroupCategory<KanjiGroup>::categories(int)'
• undefined reference to 'GroupCategory<WordGroup>::categories(int)'

Solution: Added explicit template instantiations for the categories() method in groups.cpp to ensure the linker can find the required symbols.

2. Missing Header Include

Problem: Compilation error due to missing #include <cstdint> in checked_cast.h on gcc 13.

Solution: Added the required include directive.

3. Duplicate Using Declarations

Problem: Duplicate using QComboBox::activated; declaration in zcolorcombobox.h causing compilation errors.

Solution: Commented out the duplicate declarations.

Changes Made

• groups.cpp: Added explicit template instantiations for GroupCategory<KanjiGroup> and GroupCategory<WordGroup> specializations
• checked_cast.h: Added #include <cstdint> header
• zcolorcombobox.h: Fixed duplicate using declarations
• .gitignore: Updated to exclude Qt build artifacts and generated files

Platform

Tested on: Ubuntu 22.04 with gcc 13 and Qt 5