forcePopover needs support for sourceRect

[stevengharris]

Now that user-supplied popovers can be used from the InputToolbar, they need to be presented for a specific sourceRect within the presenting View. This is done in the default LinkViewController and ImageViewController, but without support in the .forcePopover view modifier, there is no corresponding way to have user-supplied popovers presented at the selRect (i.e., the location of the selection) in the MarkupWKWebView.