workspaceThumbnail: Track later hiding drop placeholder

workspaceThumbnail: Track later hiding drop placeholder

When closing the shell during a relayout we are getting this error:

    (gnome-shell:2559407): Gjs-CRITICAL **: 06:57:21.765: Object St.Bin,
    has been already disposed — impossible to access it.
    This might be caused by the object having been destroyed from C code
    using something such as destroy(), dispose(), or remove() vfuncs.
    == Stack trace for context 0x60991609b710 ==
    #0 b   resource:///org/gnome/shell/ui/workspaceThumbnail.js:1334 (5e050636fb0 @ 17)
    #1 i   resource:///org/gnome/shell/ui/main.js:279 (30c932d102e0 @ 119)

This is because the later we added is running even when not needed (e.g
when the placeholder is not visible already), but also because we may
run the later callback after the parent has been destroyed.

Prevent this to happen, by tracking the later

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3943>