[Material] [Android, iOS] Materializing the stepper

[mattleibow]

Description of Change

Adding a renderer for the material stepper:

• refactored the actual stepper logic out so that all the controls can just implement it
• added a new renderer for material

Issues Resolved

• fixes [Material] Stepper Spec #5011
• fixes [Android] fixes TabStop on Stepper Control #5000

API Changes

• a new MaterialStepperRenderer
• a new StepperRendererManager to manage all stepper logic

Platforms Affected

• Core/XAML (all platforms)
• iOS
• Android

Behavioral/Visual Changes

• Changed to more visibility pleasing + and -
• Due to the text values being changed to a different version of +/- if a UI test is using app.Text("+") to locate the button it won't find it but if it's using app.Marked or nothing it will be able to find it just fine

Before/After Screenshots

Material (with full-width characters / new characters)

Android	iOS

Default (with full-width characters)

Android	iOS

Material (with plus and hyphen / old characters)

Android	iOS

Testing Procedure

Should just be a visual style change when using Material. Functionality should be exactly the same.

PR Checklist

• Has automated tests
• Rebased on top of the target branch at time of PR
• Changes adhere to coding standard

[mattleibow]

When testing the tabstops fix in #5000 (which I have included here because I modified the renderer code), I noticed that the Material stepper doesn't seem to be working with tabs. I am not yet sure what the cause may be, but I do see a lot of extra code in the base renderers that we may need to re-implement.

[mattleibow]

I reverted the changes to use a "Fast" renderer as it doesn't seem to work with multiple children views. I probably need to investigate this further, but for now steppers aren't the most important control.

[mattleibow]

To test the tab stops on Android, we also need #4612.

[mattleibow]

Thanks for the feedback, will get on that. In the meantime, I added the iOS one. Not sure if creating a new "control" is cool for iOS - or where I should put it.

This is not exactly what I wanted to do, since there is no need for the control - I can just do the layout in the renderer. But, I can't seem to find the concept of a no-native-view renderer. I am also thinking that this might also be good for Android.

[mattleibow]

I am not sure where we want to put this - this is very this-renderer specific and has no use elsewhere. Basically - just for layout.

[mattleibow]

This comment is still up for discussion:
#5027 (review)

[hartez]

Looks good, just have a few questions.