Implement non-blocking modal dialog using EditorWindow

Instead of using EditorUtility.DisplayDialog(), this is implemented from
EditorWindow which offers the following benefits.
* Does not block the editor window when the dialog is displayed.
* Able to display richer contents than default dialog.
* When multiple dialogs are requested to be displayed, they will be queued
  and only display one at a time.

Change-Id: Ib6393469564d64de767f015f1085ab70c2ba82e9

Author: chkuang-g

source/VersionHandlerImpl/VersionHandlerImpl.csproj

@@ -54,6 +54,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="src\Dialog.cs" />
+    <Compile Include="src\DialogWindow.cs" />
     <Compile Include="src\Logger.cs" />
     <Compile Include="src\ExecutionEnvironment.cs" />
     <Compile Include="src\EditorMeasurement.cs" />