fix(nav): ionViewCanLeave does not break navigation

[manucorporat]

Right now, the NavController does the canLeave/canEnter challenge only when the navigation change requires a transition.

1. Remove from stack
2. Evaluate if a transition is required
   • if yes: canLeave/canEnter challenge -> continue transition
   • if no: we are done

This logic leads to an important bug when canLeave or canEnter returns false (i.e. the transition has been cancelled) but the pages have been already removed from the stack.

This PR introduces a new logic (that of course passes all the current unit tests):

1. Evaluate if a transition is required
   • if yes: canLeave/canEnter challenge -> continue
   • if no: continue
2. Remove from stack
3. Evaluate if a transition is required (cached value from step one)
   • if yes: transition -> done!
   • if no: done

This way if canLeave returns false, we have not touched the stack yet.

This bug can be reproduced with the following code, the simulates returning false a couple of times, then return true:

    public trys = 0;
    ionViewCanLeave(): any {
        if (this.trys++ < 2) {
            return false;
        } else return true;
    }

It breaks the navigation 100% of the times

fixes #8408