[Cupertino] Add new CupertinoTheme class (similar to Material) and to automatically set the implicit styles

[agneszitte]

Current behavior

Cupertino does not match Material setup

Expected behavior

• Add a new CupertinoTheme class (similar to Material)
• Create explicit styles for Cupertino
• Automatically set the implicit styles
• Update the doc accordingly
  • https://github.com/unoplatform/Uno.Themes/blob/master/doc/cupertino-getting-started.md
  • https://github.com/unoplatform/Uno.Themes/blob/master/doc/cupertino-controls-styles.md

Affected platform(s):

• All platforms

[Arieldelossantos]

i've a Draft PR for the first checkmark - the CupertinoTheme class,
for the 2nd one i'm not sure of what we need to do there :P can you please give me an example @kazo0 / @agneszitte ??