Improve rendering and work around some SwiftUI bugs, e.g. with .menu
Picker on iOS (use .navigationLink instead).
Here goes the hierarchy bottom-up:
- ProviderPicker: a Picker wrapper built around ProviderManager
- ProviderContentModifier: adds a ProviderPicker on top and replaces the
content with a set of provider selectors when a provider is selected
- VPNProviderContentModifier: wrapper for ProviderContentModifier that
adds a VPN server selector
- OpenVPNView: provides a view of specific OpenVPN settings, and adds a
credentials selector to the provider/server selectors provided by
VPNProviderContentModifier
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