Fix focus handling in OverlaySheet to be managed as one FocusScope
Focus handling inside OverlaySheet had some issues, namely:
- if the contentItem was a flickable, we would be calling forceActiveFocus on the unused invisible container for non-flickables (contentItemParent)
-ScrollArea was a non-focus scope that has focus set by default, this meant it could take focus arbitrarily
This moves the FocusScope to the toplevel visible Item. Now applying "focus: true" to one of the children works as expected.