replace FSharpX.Xaml with FsXaml

will close #74

Unfortunate it currently throws an exception at `CreateRoot()`.
Maybe @ReedCopsey can look at it?

```
System.IO.IOException was unhandled by user code
  HResult=-2146232800
  Message=Assembly.GetEntryAssembly() returns null. Set the Application.ResourceAssembly property or use the pack://application:,,,/assemblyname;component/ syntax to specify the assembly to load the resource from.
  Source=PresentationFramework
  StackTrace:
       at MS.Internal.AppModel.ResourceContainer.GetResourceManagerWrapper(Uri uri, String& partName, Boolean& isContentFile)
       at MS.Internal.AppModel.ResourceContainer.GetPartCore(Uri uri)
       at System.IO.Packaging.Package.GetPartHelper(Uri partUri)
       at System.IO.Packaging.Package.GetPart(Uri partUri)
       at System.Windows.Application.GetResourceOrContentPart(Uri uri)
       at System.Windows.Application.LoadComponent(Uri resourceLocator, Boolean bSkipJournaledProperties)
       at System.Windows.Application.LoadComponent(Uri resourceLocator)
       at FsXaml.LoadXaml.from[a](String file)
       at FsXaml.XamlTypeFactory`1.CreateRoot()
       at FSharpVSPowerTools.Refactoring.UI.loadRenameDialog(RenameDialogModel viewModel) in D:\Dev\OpenSource\VisualFSharpPowerTools\src\FSharpVSPowerTools.Logic\RenameDialog.fs:line 86
       at <StartupCode$FSharpVSPowerTools-Logic>.$RenameCommandFilter.HandleRename@97-3.Invoke(Tuple`2 _arg6) in D:\Dev\OpenSource\VisualFSharpPowerTools\src\FSharpVSPowerTools.Logic\RenameCommandFilter.fs:line 107
       at <StartupCode$FSharpVSPowerTools-Logic>.$RenameCommandFilter.HandleRename@91-2.Invoke(Tuple`2 _arg5) in D:\Dev\OpenSource\VisualFSharpPowerTools\src\FSharpVSPowerTools.Logic\RenameCommandFilter.fs:line 91
       at <StartupCode$FSharpVSPowerTools-Logic>.$RenameCommandFilter.HandleRename@90-1.Invoke(DocumentState _arg4) in D:\Dev\OpenSource\VisualFSharpPowerTools\src\FSharpVSPowerTools.Logic\RenameCommandFilter.fs:line 90
       at <StartupCode$FSharpVSPowerTools-Logic>.$RenameCommandFilter.HandleRename@89.Invoke(Unit unitVar) in D:\Dev\OpenSource\VisualFSharpPowerTools\src\FSharpVSPowerTools.Logic\RenameCommandFilter.fs:line 89
       at FSharpVSPowerTools.MaybeBuilder.Delay[T](FSharpFunc`2 f) in D:\Dev\OpenSource\VisualFSharpPowerTools\src\FSharpVSPowerTools.Core\Utils.fs:line 55
       at FSharpVSPowerTools.Refactoring.RenameCommandFilter.HandleRename() in D:\Dev\OpenSource\VisualFSharpPowerTools\src\FSharpVSPowerTools.Logic\RenameCommandFilter.fs:line 88
       at FSharpVSPowerTools.Refactoring.RenameCommandFilter.Microsoft-VisualStudio-OLE-Interop-IOleCommandTarget-Exec(Guid& pguidCmdGroup, UInt32 nCmdId, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut) in D:\Dev\OpenSource\VisualFSharpPowerTools\src\FSharpVSPowerTools.Logic\RenameCommandFilter.fs:line 148
       at Microsoft.VisualStudio.Editor.Implementation.CommandChainNode.InnerExec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut)
       at Microsoft.VisualStudio.Editor.Implementation.CommandChainNode.Exec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut)
       at FSharpVSPowerTools.XmlDoc.XmlDocFilter.Microsoft-VisualStudio-OLE-Interop-IOleCommandTarget-Exec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut) in D:\Dev\OpenSource\VisualFSharpPowerTools\src\FSharpVSPowerTools.Logic\XmlDocFilter.fs:line 75
       at Microsoft.VisualStudio.Editor.Implementation.CommandChainNode.InnerExec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut)
       at Microsoft.VisualStudio.Editor.Implementation.CommandChainNode.Exec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut)
       at FSharpVSPowerTools.StandardCommandDispatcher.Exec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut) in d:\Dev\OpenSource\VisualFSharpPowerTools\src\FSharpVSPowerTools\StandardCommandDispatcher.cs:line 103
       at Microsoft.VisualStudio.Editor.Implementation.CommandChainNode.InnerExec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut)
       at Microsoft.VisualStudio.Editor.Implementation.CommandChainNode.InnerExec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut)
       at Microsoft.VisualStudio.Editor.Implementation.SimpleTextViewWindow.Exec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut)
       at Microsoft.VisualStudio.Editor.Implementation.CompoundTextViewWindow.Exec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut)
       at Microsoft.VisualStudio.Platform.WindowManagement.DocumentObjectSite.Exec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut)
       at Microsoft.VisualStudio.Platform.WindowManagement.WindowFrame.Exec(Guid& pguidCmdGroup, UInt32 nCmdID, UInt32 nCmdexecopt, IntPtr pvaIn, IntPtr pvaOut)
  InnerException: 
```