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/Main/MacOSXSecureTextFieldHotkeys.h

@@ -0,0 +1,20 @@
+/*
+ Copyright (c) 2013-2025 AM Crypto. All rights reserved.
+
+ Governed by the Apache License 2.0 the full text of which is
+ contained in the file License.txt included in VeraCrypt binary and source
+ code distribution packages.
+*/
+
+#ifndef TC_HEADER_Main_MacOSXSecureTextFieldHotkeys
+#define TC_HEADER_Main_MacOSXSecureTextFieldHotkeys
+
+#ifdef TC_MACOSX
+namespace VeraCrypt
+{
+	void InstallMacOSXSecureTextFieldHotkeys ();
+	void UninstallMacOSXSecureTextFieldHotkeys ();
+}
+#endif
+
+#endif // TC_HEADER_Main_MacOSXSecureTextFieldHotkeys