[guilib] Ensure ListItems release the layout before it is destroyed

[enen92]

Description

This PR makes sure listitems in a grouplist release the reference to the container/layout before it is destroyed. Right now this occurs too late, making the container pointer invalid when the pointed object is deleted (as a child of a group).
Credits for the proper solution goes to @peak3d. Thanks also to @fritsch and @pkerling for the patience :)

Motivation and Context

Fix #15226 and make we still have extended.info support in Estuary for v18 (invalidating #15258).

How Has This Been Tested?

Compiled and tested on osx

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

[ronie]

tested and it works ok on my end. thx!

[yol]

Looks innocent, but it's really hard to say if this will break anything else or not

[peak3d]

@pkerling FreeMemory simply removes the Layouts which are used for presenting the listItem.
There is no real risk, because layouts are not a shared resource over GUI control parents

Edit: But yes, the missing blank should be inserted :-)

[MartijnKaijser]

Shall we merge and see for couple of days?

[enen92]

@MartijnKaijser have been using this for a few days and haven't found any regression yet. Peak3d did a lot of rework on GUI for v18 and this fix is proposed by him anyways (as the bug was introduced in his rework PR). I'd say this is safe enough to be merged. We can always revert if justified.