[Estuary] fix navigation in skinsettings

[ronie]

Description

this fixes a navigation issue in Skin Settings > Main menu items
when navigating back from the scrollbar to the options grouplist would focus the wrong item.

this fixes the problem by pointing onleft/onright to the actual grouplist, instead of the parent group.

Motivation and Context

bug report on the forum https://forum.kodi.tv/showthread.php?tid=262373&pid=2978180#pid2978180

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed