Fix the BackgroundColor to Background update

[mattleibow]

Description of Change

The reason setting the BackgroundColor of a Page did not update the Background was due to the "inheritance" of the handlers. The PageHandler effectively inherits from ViewHandler, however, we remap some things in Controls and this caused an issue because we used the wrong type.

The VisualElement.RemapForControls assumed that all VisualElements are Views, and this is not true for the case of Page. However, this is true for the case of IView.

Issues Fixed

• Fixes [Bug] BackgroundColor of ContentPage not being updated on AppThemeBinding update #5681
• Fixes AppThemeBinding not working (p11/p12) #4335
• Fixes Custom theme ResourceDictionary not working #4143
• Fixes [Android 11] Behavior does not modify properties of attached object when modification occurs in an EventHandler #4516
• Fixes BackgroundColor not changed #5380

[mattleibow]

I see @jsuarezruiz did this, but in another way: #5706

Not sure if changing to IView is good or bad? @PureWeen ?

[mattleibow]

This is the actual fix: View -> IView

[StephaneDelcroix]

no tests ?

[mattleibow]

Still working on that.

[rachelkang]

I changed the Page BackgroundColor to Background in the template styles - should we change that back with this fix? Or should either work fine now?

[PureWeen]

@rachelkang I would change it back. Since setting Background makes BackgroundColor unusable it's unfortunately confusing if we're setting Background in our styles and they go to set BackgroundColor

[javiercn]

Blazor related changes look good.

Can't comment on other changes!

[mattleibow]

oops for the IPicker :(

[pedromcrocha]

in https://github.dev/dotnet/maui/blob/main/src/Controls/src/Core/Compatibility/Handlers/VisualElementRenderer.cs
around line 175
where it is
#if IOS
protected virtual void SetBackgroundColor(Color? color)
#else
protected virtual void UpdateBackgroundColor()
#endif
{
if (Element != null)
ViewHandler.MapBackground(this, Element);
}

shouldn't be?

#if IOS
protected virtual void SetBackgroundColor(Color? color)
#else
protected virtual void UpdateBackgroundColor()
#endif
{
if (Element != null)
ViewHandler.MapBackgroundColor(this, Element);
}