Update CMake include directories for improved modularity and organization

- Refactored target_include_directories in app.cmake, emu.cmake, and z3ed.cmake to use absolute paths for better clarity and maintainability.
- Updated test_manager.cc and test_manager.h to simplify ImGui header inclusion.
- Enhanced test CMakeLists.txt to conditionally include directories for the extract_vanilla_values utility, ensuring proper integration with the new structure.

src/app/test/test_manager.cc

@@ -6,7 +6,7 @@
 #include "app/core/platform/file_dialog.h"
 #include "app/gfx/arena.h"
 #include "app/gui/icons.h"
-#include "imgui/imgui.h"
+#include "imgui.h"
 #include "util/log.h"
 
 // Forward declaration to avoid circular dependency