monitor-manager: Notify privacy screen changes on hotkey press

When privacy screen is changed and this happens on explicit user request
(that is not a setting change) we should notify about this via an OSD.

To perform this, we keep track of the reason that lead to a privacy
screen change, and when we record it we try to notify the user about.

When the hardware has not an explicit hotkey signal but we record a
change we must still fallback to this case.

Fixes: #2105
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1952>

src/backends/meta-monitor-manager-private.h

@@ -35,6 +35,7 @@
 #include "backends/meta-viewport-info.h"
 #include "core/util-private.h"
 #include "meta/display.h"
+#include "meta/meta-enum-types.h"
 #include "meta/meta-monitor-manager.h"
 
 #define META_MONITOR_MANAGER_MIN_SCREEN_WIDTH 640
@@ -62,6 +63,14 @@ typedef enum _MetaLogicalMonitorLayoutMode
   META_LOGICAL_MONITOR_LAYOUT_MODE_PHYSICAL = 2
 } MetaLogicalMonitorLayoutMode;
 
+/* The source the privacy screen change has been triggered */
+typedef enum
+{
+  META_PRIVACY_SCREEN_CHANGE_STATE_NONE,
+  META_PRIVACY_SCREEN_CHANGE_STATE_PENDING_HOTKEY,
+  META_PRIVACY_SCREEN_CHANGE_STATE_PENDING_SETTING,
+} MetaPrivacyScreenChangeState;
+
 /*
  * MetaCrtcAssignment:
  *
@@ -149,6 +158,8 @@ struct _MetaMonitorManager
   GnomePnpIds *pnp_ids;
 
   MetaMonitorSwitchConfigType current_switch_config;
+
+  MetaPrivacyScreenChangeState privacy_screen_change_state;
 };
 
 /**
@@ -440,4 +451,6 @@ MetaViewportInfo * meta_monitor_manager_get_viewports (MetaMonitorManager *manag
 
 GList * meta_monitor_manager_get_virtual_monitors (MetaMonitorManager *manager);
 
+void meta_monitor_manager_maybe_emit_privacy_screen_change (MetaMonitorManager *manager);
+
 #endif /* META_MONITOR_MANAGER_PRIVATE_H */