backlight panel: flash on hotkey changes

modules/Backlight.qml

@@ -13,7 +13,21 @@ M.BarSection {
 
     property int percent: 0
     property bool _panelHovered: false
-    readonly property bool _showPanel: root._hovered || _panelHovered
+    property bool _osdActive: false
+    readonly property bool _showPanel: root._hovered || _panelHovered || _osdActive
+
+    onPercentChanged: if (percent > 0) _flashPanel()
+
+    function _flashPanel() {
+        _osdActive = true;
+        _osdTimer.restart();
+    }
+
+    Timer {
+        id: _osdTimer
+        interval: 1500
+        onTriggered: root._osdActive = false
+    }
 
     Process {
         id: adjProc