PaneItemExpander deeper nesting

[ignatz]

I'd like to nest pane items more deeply than 1 level. The implementation clearly asserts: "There can not be nested PaneItemExpanders" while MS' documentation reads: "Although NavigationViewItem can contain any number of nested levels, we recommend keeping your app's navigation hierarchy shallow. We believe two levels is ideal for usability and comprehension." (https://learn.microsoft.com/en-us/windows/apps/design/controls/navigationview#hierarchical-navigation).

Describe the solution you'd like I'm mostly wondering if the limiting is intentional, if there's a technical reason, or if you'd be willing to accept a patch.

[bdlukaa]

Because of the way we traverse the pane items and calculate the index, nesting pane items is not allowed.

A patch would be appreciated, but I recommend you to read #919 first!