`FeatureFormView` & `PopupView` - Add ability to hide default headers

[dfeinzimer]

Closes #835

• Adds:
  • PopupView.popupHeader(_:)
  • Controls the visibility of the title and close button (visible by default).
  • FeatureFormView.formHeader(_:)
  • Controls the visibility of the title (visible by default).
• Deprecates:
  • PopupView.showCloseButton(_:)

[dfeinzimer]

@mhdostal and @rolson this is ready for review once again. The updated changes are:

• Adds:
  • PopupView.popupHeader(_:)
  • Controls the visibility of the title and close button (visible by default).
  • FeatureFormView.formHeader(_:)
  • Controls the visibility of the title (visible by default).
• Deprecates:
  • PopupView.showCloseButton(_:)

In regards to the suggestion above, would it be better to avoid the complication of maintaining new initializers? With these changes devs can hide the header and easily include a custom one with a VStack:

.sheet(isPresented: $showPopup) { [popup] in
    VStack(alignment: .leading) {
        Text(popup!.title)
            .font(.title)
        PopupView(popup: popup!)
            .popupHeader(.hidden)
    }
    .padding()
}

[dfeinzimer]

This is still ready for review. I made one more slight change to avoid breaking the behavior of PopupView.showCloseButton(_:).