fix: use positionViewAtBeginning instead of set contentY

It is not recommended to use contentX or contentY to position the
view at a particular index. This is unreliable since removing items
from the start of the list does not cause all other items to be
repositioned, and because the actual start of the view can vary based
on the size of the delegates.

PMS: BUG-356795
Log:

Author: BLumia

qml/windowed/AppListView.qml

@@ -24,7 +24,7 @@ FocusScope {
         let wasFollowing = listView.highlightFollowsCurrentItem
         listView.highlightFollowsCurrentItem = false
         listView.currentIndex = 0
-        listView.contentY = 0
+        listView.positionViewAtBeginning()
         listView.highlightFollowsCurrentItem = wasFollowing
         if (!LauncherController.visible) {
             alphabetCategoryPopup.close()