[Lock screen] Make clearPassword() do what it says and then use it

Right now the clearPassword() signal does not actually clear the
password; it only selects all text. This is a violation of its name
as well as being pointless since there's no longer a way to see the
unmasked text, which means you always need to re-enter the whole
password anyway.

BUG: 412252
FIXED-IN: 5.20

lookandfeel/contents/lockscreen/LockScreenUi.qml

@@ -64,7 +64,7 @@ PlasmaCore.ColorScope {
     Connections {
         target: sessionManagement
         onAboutToSuspend: {
-            mainBlock.mainPasswordBox.text = "";
+            root.clearPassword();
         }
     }
 
@@ -121,7 +121,7 @@ PlasmaCore.ColorScope {
                 inputPanel.showHide();
             }
             if (!uiVisible) {
-                mainBlock.mainPasswordBox.text = "";
+                root.clearPassword();
             }
         }
         Keys.onPressed: {

lookandfeel/contents/lockscreen/MainBlock.qml

@@ -89,7 +89,7 @@ SessionManagementScreen {
                 target: root
                 onClearPassword: {
                     passwordBox.forceActiveFocus()
-                    passwordBox.selectAll()
+                    passwordBox.text = "";
                 }
             }
         }