Fix Avalonia Obsolete Warnings

[christophwille]

Reported via #82 (comment)

[christophwille]

@jeffreye we should consider fixing those soonish because who knows which Avalonia version will turn obsoletions into actual build breaks.

2>D:\GitWorkspace\AvaloniaILSpy\SharpTreeView\EditTextBox.cs(21,27,21,44): warning CS0672: Member 'EditTextBox.OnTemplateApplied(TemplateAppliedEventArgs)' overrides obsolete member 'TemplatedControl.OnTemplateApplied(TemplateAppliedEventArgs)'. Add the Obsolete attribute to 'EditTextBox.OnTemplateApplied(TemplateAppliedEventArgs)'.
2>D:\GitWorkspace\AvaloniaILSpy\SharpTreeView\EditTextBox.cs(34,8,34,17): warning CS0108: 'EditTextBox.SelectAll()' hides inherited member 'TextBox.SelectAll()'. Use the new keyword if hiding was intended.
2>D:\GitWorkspace\AvaloniaILSpy\SharpTreeView\SharpTreeView.cs(98,47,98,74): warning CS0108: 'SharpTreeView.IsTextSearchEnabledProperty' hides inherited member 'SelectingItemsControl.IsTextSearchEnabledProperty'. Use the new keyword if hiding was intended.
2>D:\GitWorkspace\AvaloniaILSpy\SharpTreeView\SharpTreeView.cs(101,15,101,34): warning CS0108: 'SharpTreeView.IsTextSearchEnabled' hides inherited member 'SelectingItemsControl.IsTextSearchEnabled'. Use the new keyword if hiding was intended.
2>D:\GitWorkspace\AvaloniaILSpy\SharpTreeView\SharpTreeView.cs(768,15,768,26): warning CS0108: 'SharpTreeView.UnselectAll()' hides inherited member 'ListBox.UnselectAll()'. Use the new keyword if hiding was intended.
2>D:\GitWorkspace\AvaloniaILSpy\SharpTreeView\EditTextBox.cs(23,4,23,29): warning CS0618: 'TemplatedControl.OnTemplateApplied(TemplateAppliedEventArgs)' is obsolete: 'Use OnApplyTemplate'
2>D:\GitWorkspace\AvaloniaILSpy\SharpTreeView\SharpTreeViewItem.cs(60,8,60,21): warning CS0618: 'PointerPressedEventArgs.MouseButton' is obsolete: 'Use PointerPressedEventArgs.GetCurrentPoint(this).Properties'
2>D:\GitWorkspace\AvaloniaILSpy\SharpTreeView\SharpTreeViewItem.cs(64,9,64,21): warning CS0618: 'PointerPressedEventArgs.ClickCount' is obsolete: 'Use DoubleTapped event or Gestures.DoubleRightTapped attached event'

3>------ Build started: Project: ILSpy.Core, Configuration: Debug Any CPU ------
3>D:\GitWorkspace\AvaloniaILSpy\ILSpy.Core\Controls\DockedPane.cs(38,33,38,50): warning CS0672: Member 'DockedPane.OnTemplateApplied(TemplateAppliedEventArgs)' overrides obsolete member 'TemplatedControl.OnTemplateApplied(TemplateAppliedEventArgs)'. Add the Obsolete attribute to 'DockedPane.OnTemplateApplied(TemplateAppliedEventArgs)'.
3>D:\GitWorkspace\AvaloniaILSpy\ILSpy.Core\TextView\DecompilerTextView.xaml.cs(796,69,796,116): warning CS0618: 'IPointerDevice.GetPosition(IVisual)' is obsolete: 'Use PointerEventArgs.GetPosition'
3>D:\GitWorkspace\AvaloniaILSpy\ILSpy.Core\Controls\DockedPane.cs(40,13,40,38): warning CS0618: 'TemplatedControl.OnTemplateApplied(TemplateAppliedEventArgs)' is obsolete: 'Use OnApplyTemplate'
3>D:\GitWorkspace\AvaloniaILSpy\ILSpy.Core\MainWindow.xaml.cs(437,33,437,56): warning CS0618: 'NativeMenuItemSeperator' is obsolete: 'This class exists to maintain backwards compatibility with existing code. Use NativeMenuItemSeparator instead'