Decompose Button-like components to separate parts

[PavelHolec]

Components could be decomposed to separate public parts for better reusability and flexibility. We would still keep the convenience standalone component, but it would be a thin wrapper over its FooButtonStyle + FooContent implementation.

Add internal environmentKey for buttonPressed(Bool) to be able to snapshot and preview active button states.

For example, ListChoice would be restructured into:

• Convenience component wrapper ListChoice(…), as-is
• Content component ListChoiceContent that defines the layout itself (plus accessibility)
• A ButtonStyle ListChoiceButtonStyle that takes care of proper isPressed and similar interaction (keeping title as a Button Label to provide label accessibility automatically)

This way, such components could be structured as:

ListChoice(...)

==

SwiftUI.Button(...) {
  ListChoiceContent(..)
}
.buttonStyle(ListChoiceButtonStyle(..))

Limitation

In SwiftUI, a ButtonStyle definition cannot be specified using other (more generic, for example) .buttonStyle()