Interactive mode on tooltips only works when opened for the first time.

[Dániel Kis-Nagy]

When closing the tooltip, the reference required for interactive mode to function isn't cleared, resulting in interactive mode being effectively disabled when subsequently opening the tooltip again. This can be reproduced by opening https://ng-lightning.github.io/ng-lightning/#/components/tooltips, hovering over the interactive tooltip, moving the mouse away to close it, and then hovering again to open it the second time; this time, the interactive behavior won't kick in.

PR with a suggested fix will follow soon.