macOS: Fix Command-A in password fields

Install a macOS-specific secure text field hotkey handler so Command-A selects the full contents of password controls when Cocoa does not route the shortcut through wxWidgets accelerators. Keep the existing wxWidgets accelerator handler for Command-V and Command-A, and recognize the standard paste/select-all IDs when they do reach the C++ event path.

Add Objective-C++ compilation support for the macOS helper and include it in the GUI target only on macOS.

Fixes https://github.com/veracrypt/VeraCrypt/issues/1567

src/Build/Include/Makefile.inc

@@ -51,7 +51,13 @@ clean:
 %.o: %.cpp
 	@echo Compiling $(<F)
 	$(CXX) $(CXXFLAGS) -c $< -o $@
-	
+
+ifeq "$(PLATFORM)" "MacOSX"
+%.o: %.mm
+	@echo Compiling $(<F)
+	$(CXX) $(CXXFLAGS) -x objective-c++ -fblocks -c $< -o $@
+endif
+
 %.osse41: %.cpp
 	@echo Compiling $(<F)
 	$(CXX) $(CXXFLAGS) -mssse3 -msse4.1 -c $< -o $@