Use a transparent Modal Page for Dialogs

[dansiegel]

Description of Change

Dialogs up until now relied on reparenting the content on the currently visible page. While this was an elegant way of getting customizable and MVVM friendly dialogs into your Xamarin.Forms app, it also had some negative side affects including:

• Forced a full UI/Binding refresh
• The content was only within the scope of the currently visible ContentPage allowing full UI interaction with Navigation Bars and the MasterDetailPage Menu

NOTE There are no unit tests for this as it is ultimately a change to the UI

Android	iOS
	*Known issue on iOS xamarin/Xamarin.Forms#11877

Bugs Fixed

• fixes [Enhancement] DialogService prevent switch tabs on TabbedPage #1892
• resolves Update the IDialogService to use a Modal Page with Transparent Background #2068
• fixes Issue with showing a dialog on a page that uses CollectionView in UWP (Prism.Forms).  #2132

API Changes

none

Behavioral Changes

Instead of reparenting we now put the Dialog content inside of a new ContentPage with a transparent background. This allows us to present the new Dialog Page modally over the current page using the same masking and content that you had before however the content will now fully cover the NavigationPage Title bar and TabbedPage Tab bar, as well as prevent accessing a MasterDetail while the dialog is presented.

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard