Cannot attach GtkPopover to GtkTreeView

Apparently it will mess up the CSS nodes in some weird way. Somewhat annoying for the cases where we still inherit from GtkTreeView (like Builder's project tree).

It looks like GTK 4's filechooser gets around this by attaching the popover to the GtkFileChooserWidget to do placement (still somewhat incorrectly).

Gtk[ 194119]: CRITICAL: gtk_css_node_insert_after: assertion 'previous_sibling == NULL || previous_sibling->parent == parent' failed