bug: VitaminButtons do not work properly with iOS15 plain button

[daniel-dumortier]

Describe the bug When you drag a button in a storyboard in Xcode 13+, it is automatically forced with the 'Plain' configuration. With this configuration, the styling of VitaminButton does not work well (especially the bacjground color)

Steps to reproduce

1. add a Button to your storyboard
2. check in the attribute inspector that its "Style" property is set to anything else than Default (it should be Plain)
3. programmatically assign a .primary Vitmain style to this button
4. run your app
5. button will have no background

Expected behavior the button should have a coloured background

Screenshots

Device (please complete the following information):

• Model: iPhone 8 (but that does not depend on the phone)
• OS: iOS
• Version: 15.4