macOS: add screen protection preference

Persist a macOS-only preference for disabling wx content protection while keeping protection enabled by default.

Add a Preferences checkbox that reuses IDC_DISABLE_SCREEN_PROTECTION and reapply the protection state after preference changes.

Apply content-protection updates to all current top-level wx windows so changing the preference while a modal dialog is active does not leave the main frame or other windows with stale protection state.

src/Main/UserPreferences.h

@@ -33,6 +33,7 @@ namespace VeraCrypt
 			CloseSecurityTokenSessionsAfterMount (false),
 			EMVSupportEnabled (false),
 			DisableKernelEncryptionModeWarning (false),
+			DisableScreenProtection (false),
 			DismountOnInactivity (false),
 			DismountOnLogOff (true),
 			DismountOnPowerSaving (false),
@@ -76,6 +77,7 @@ namespace VeraCrypt
 		bool CloseSecurityTokenSessionsAfterMount;
         bool EMVSupportEnabled;
 		bool DisableKernelEncryptionModeWarning;
+		bool DisableScreenProtection;
 		bool DismountOnInactivity;
 		bool DismountOnLogOff;
 		bool DismountOnPowerSaving;