closeDialog: Remove transitions before resetting dialog

On X11, _onFocusChanged() updates the input region, as well as the reactive-ness of the dialog's buttons.

That method is not only used as signal handlers (which are correctly disconnected when the dialog is hidden), it also runs when the "show" transition completes.

That's a problem if the transition is still ongoing when the dialog is hidden, as it will then only complete when it is replaced by the "hide" transition, after the this._dialog has been reset to null, and trying to access the dialog's buttons results in an error.

Avoid this by explicitly removing all transition on hide before resetting the dialog.

Related #2467 (closed)