[lookandfeel] Avoid rendering invisible contents

An opacity of 0 but still visible still results in nodes in the
scenegraph, which is wasteful.  This is shown in gammaray with some
warnings.

Enabled is also bound to visible as if a text field has focus it still
animates the cursor icon even if inivisble, producing wakeups.

BUG: 347772
FIXED-IN: 5.21

lookandfeel/contents/lockscreen/LockScreenUi.qml

@@ -228,6 +228,12 @@ PlasmaCore.ColorScope {
             height: lockScreenRoot.height + units.gridUnit * 3
             focus: true //StackView is an implicit focus scope, so we need to give this focus so the item inside will have it
 
+            // 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
                 lockScreenUiVisible: lockScreenRoot.uiVisible