UWP support, and get user info

[JogyBlack]

Changes:

• Added UWP support.
• Added code to pre-populate the sender name with the currently logged user on Windows or Android. Based on the article https://doumer.me/get-the-device-current-user-name-with-xamarin-forms/, updated to correctly request permissions to access the user account on newer Android versions.

[JogyBlack]

Screenshot of the UWP version, with the sender name automatically populated from the current user information.

[codemillmatt]

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[codemillmatt]

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