UI: Fix visibility and lock checkbox styling on system theme

In 7931f2a the classes have been
removed in favor of properties, but this styling change appears to have
been reverted by accident, likely by the theming overhaul.

UI/data/themes/System.obt

@@ -213,31 +213,31 @@ OBSBasicSettings QListWidget::item {
 
 /* Locked CheckBox */
 
-LockedCheckBox {
+QCheckBox[lockCheckBox=true] {
     outline: none;
     background: transparent;
 }
 
-LockedCheckBox::indicator:checked {
+QCheckBox[lockCheckBox=true]::indicator:checked {
     image: url(:res/images/locked.svg);
 }
 
-LockedCheckBox::indicator:unchecked {
+QCheckBox[lockCheckBox=true]::indicator:unchecked {
     image: url(:res/images/unlocked.svg);
 }
 
 /* Visibility CheckBox */
 
-VisibilityCheckBox {
+QCheckBox[visibilityCheckBox=true] {
     outline: none;
     background: transparent;
 }
 
-VisibilityCheckBox::indicator:checked {
+QCheckBox[visibilityCheckBox=true]::indicator:checked {
     image: url(:res/images/visible.svg);
 }
 
-VisibilityCheckBox::indicator:unchecked {
+QCheckBox[visibilityCheckBox=true]::indicator:unchecked {
     image: url(:res/images/invisible.svg);
 }