Improve design with Shell

[Wenfengcheng]

Improve design with Shell.

More feedback

• I've been struggling to remove safeAreaInsets on MainPage on iOS until I find out this bug:Xamarin.Forms/issues/6499, which not fixed until latest version.
• I use Shell.Current.GoToAsync("//picture"); to navigate page, but when I use Shell.Current.Navigation.PopModalAsync(); to pop back, it crashed! Maybe GoToAsync method not effect navigation stack.bug:Xamarin.Forms/issues/4769
• So I use Shell.Current.GoToAsync("//main"); to pop back, but I navigate to picture page next time, this page is not recreated which means the page instance and viewmodel not disposed.

[codemillmatt]

Thanks for completing the challenge! Fill out this form so we can send you the swag! https://aka.ms/cog-svcs-swag

[Wenfengcheng]

Thanks for completing the challenge! Fill out this form so we can send you the swag! https://aka.ms/cog-svcs-swag

Thank you!