For some reason on macOS windows that are activated are not appearing on
top, and get lost behind other application windows. The platform
implementation on macOS is seemingly correctly calling the
`makeKeyAndOrderFront` native windowing API, but it's not being made
"key and front".

Setting the "topmost" flag true (and then false again) brings our
windows to the front, which is what we want.

Include the images resource across all the Avalonia UI applications.

Add a generic credentials prompt view model and command for collecting
username/passwords.

Add an Avalonia-based UI prompt for the generic/basic credential prompt.

Add a WPF/Windows UI prompt for the generic/basic username and password
prompt.

Replace calls to the native system UI prompts with shelling out to the
generic helper UI.