[Enhancement] Add ToolTip and ToolTipPlacement properties

[mhrastegari]

Summary

The idea is from #11306

In Xamarin.Forms it is not yet possible to display ToolTips. These are small labels with information for the user, which are displayed when the user presses a view for a short time.

+and just like radio button the macOS, UWP, Android does natively supporting it but iOS doesn't :)

API Changes

• Add a property named ToolTip of type string and add it to VisualElement class.
• Add a property named ToolTipPlacement of type ToolTipPlacement and add it to VisualElement class.
• Add an enum named ToolTipPlacement:

  public enum ToolTipPlacement
    {
        Below = 0,
        Above = 1,
        Left = 2,
        Right = 3,
        TopLeft = 4,
        TopRight = 5,
        BottomLeft = 6,
        BottomRight = 7
    }

• Add platform specific implementations.

Examples

<BoxView ToolTip="More options"
         ToolTipPlacement="Below"></BoxView>

or

<ImageButton Source="Mail.png"
             BackgroundColor="Pink"
             ToolTip="Send an email"
             ToolTipPlacement="Above"></ImageButton>

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.