Win8 compatibility

[eltos]

Changes:

• Make Microsoft.Toolkit.Uwp notifications optional

  Microsoft.Toolkit.Uwp requires Windows version 1809 (build 17763) or higher To ensure compatibility with older windows versions, exceptions with UWP notifications are caught. Notifications are not important for the functionality.

• Fix ObjectDisposedException due to Close() call in constructor

  Exception was thrown after pasting in autosave mode completed and the app was closing down

• Use platform independent HttpClient

  i.e. System.Net.Http instead of Windows.Web.Http

Closes #32

Testing

Tested by @wtfiwinomgs on Windows 8