[HxSidebarItem] Allow navigation on expandable items

[crdo]

In some rare cases, after clicking on parent SidebarItem, we need to do navigation + expanding child nodes. In Bootstrap, handling of href on parent a is prevented, so I suggest to add OnClick parameter as a work-around.

[hakenr]

@crdo Instead of OnClick (extending the API and allowing all the possible misinterpretations of the behavior), I tried to make the Href link work by executing the navigation action manually when needed.

I will publish the proposed solution in a Havit.Blazor prerelease and let's give it a try.