Honor the positioning Rectangle passed to Popover.Show

If the passed rectangle is Rectangle.Zero, it defaults to the screen bounds of the reference Widget.
Otherwise, it represents an area inside the reference Widget relative to which the popover will be shown.
[Wpf] SpinButton was before created with Xaml which MonoDevelop can't handle so now i rewrited to C#
Merge pull request #216 from DavidKarlas/wpfSpinButtonFromXamlToCsharp

[Wpf] SpinButton rewritten to C#
Merge branch 'master' into desired-size