The tour system is based on many assumptions, some of those not being
true all the time.  In practice, it works most of the time, but when
some slight changes occurs, we may observe undeterministic behavior.
This was actually the case in a dev branch working on assets: the order
of some files was changed, and it caused crashes in various tours.

This helped us identify two issues in the Tip class:

- the Tip class may be destroyed before it is completely started (so,
  the destroy method should not assume that the code in start was called
- the attach_to method should not resolve if the tip was destroyed
meanwhile.  This is critical, because some code uses that promise to act
on the widget.

This commit fixes both of these issues.

closes odoo/odoo#67872

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>