feat: Add QR code generation functionality to PrivateBin API library

- Add generate_qr_code() function for creating QR codes from PrivateBin links
- Implement SVG output format for scalable QR code generation
- Add qr_generator.h/cpp with self-contained QR code implementation
- Update CMakeLists.txt to include new QR code source files
- Integrate QR code documentation into README.md
- Update example program to demonstrate QR code generation
- Update CHANGELOG.md with v0.1.1.6 release notes
- Remove separate README_QR_CODE.md file

The implementation provides simplified QR code generation with:
- Position detection patterns for QR code recognition
- Configurable size and border parameters
- No external dependencies
- Comprehensive error handling
- Memory management using existing free_string() function

include/privatebinapi.h

@@ -76,6 +76,18 @@ PRIVATEBIN_API int get_paste(const char* server_url, const char* paste_id,
 PRIVATEBIN_API int delete_paste(const char* server_url, const char* paste_id, 
                                const char* delete_token);
 
+/**
+ * Generates a QR code for a PrivateBin link
+ * 
+ * @param url The PrivateBin URL to encode in the QR code
+ * @param qr_code_data Output parameter for the QR code data (SVG format)
+ * @param size The size of the QR code in pixels (default: 256)
+ * @param border The border size around the QR code in modules (default: 4)
+ * @return 0 on success, error code on failure
+ */
+PRIVATEBIN_API int generate_qr_code(const char* url, char** qr_code_data, 
+                                   int size, int border);
+
 /**
  * Frees memory allocated by the API functions
  *