[screenlocker] Clear password field after an idle timeout

Clear the password field if no input happened for 30 sec. This is
intended for cases that the user starts entering the password and
then moves away from the system leaving it in a state where anyone
could unlock the session by just pressing enter.

BUG: 326633
FIXED-IN: 5.3.0
REVIEW: 122511

lookandfeel/contents/lockscreen/LockScreen.qml

@@ -164,6 +164,13 @@ Image {
                                 }
                                 text = "";
                             }
+                            onTextChanged: {
+                                if (text == "") {
+                                    clearTimer.stop();
+                                } else {
+                                    clearTimer.restart();
+                                }
+                            }
 
                             Keys.onLeftPressed: {
                                 if (text == "") {
@@ -179,6 +186,14 @@ Image {
                                     event.accepted = false;
                                 }
                             }
+                            Timer {
+                                id: clearTimer
+                                interval: 30000
+                                repeat: false
+                                onTriggered: {
+                                    passwordInput.text = "";
+                                }
+                            }
                         }
 
                         PlasmaComponents.Button {