[lookandfeel] Fix wake existing screensaver mode with key presses

In commit 45e0a722fb we changed the focus
on the textfield with the intention of fixing the cusor blinking.

This had the unfortunate side effect of breaking waking out of the
screensaver mode with just the keyboard.

That screensaver mode was bound to:
```
property bool blockUI: mainBlock.mainPasswordBox.text.length > 0
```
for that to change, we need to have focus.

Instead this patches cursor blinking more explicity.

Doing it only in the lock screen (as opposed to the toolkit) makes sense
as this only happens due to somewhat unusual usage inside the
lockscreen.

BUG: 431286

lookandfeel/contents/lockscreen/LockScreenUi.qml

@@ -230,9 +230,6 @@ PlasmaCore.ColorScope {
 
             // this isn't implicit, otherwise items still get processed for the scenegraph
             visible: opacity > 0
-            // changing enabled will toggle if an item can have activeFocus, which otherwise
-            //keeps the text cursor blinking even when invisble
-            enabled: visible
 
             initialItem: MainBlock {
                 id: mainBlock

lookandfeel/contents/lockscreen/MainBlock.qml

@@ -66,6 +66,12 @@ SessionManagementScreen {
             enabled: !authenticator.graceLocked
             revealPasswordButtonShown: true
 
+            // In Qt this is implicitly active based on focus rather than visibility
+            // in any other application having a focussed invisible object would be weird
+            // but here we are using to wake out of screensaver mode
+            // We need to explicitly disable cursor flashing to avoid unnecessary renders
+            cursorVisible: visible
+
             onAccepted: {
                 if (lockScreenUiVisible) {
                     startLogin();