A tree control. Fix scrolling.

Isolate the treeview into its own TreeControl file so everyone can use
it. Fix scrolling while we're at it. Context menus still need to be
implemented.

Author: shana

src/GitHub.Api/Git/GitBranch.cs

@@ -2,7 +2,7 @@
 
 namespace GitHub.Unity
 {
-    interface ITreeData
+    public interface ITreeData
     {
         string Name { get; }
         bool IsActive { get; }

src/UnityExtension/Assets/Editor/GitHub.Unity/GitHub.Unity.csproj

@@ -103,6 +103,7 @@
     <Compile Include="UI\LoadingView.cs" />
     <Compile Include="UI\PublishView.cs" />
     <Compile Include="UI\InitProjectView.cs" />
+    <Compile Include="UI\TreeControl.cs" />
     <Compile Include="UI\UserSettingsView.cs" />
     <Compile Include="UI\GitPathView.cs" />
     <Compile Include="UI\SettingsView.cs" />
@@ -217,4 +218,4 @@
   </Target>
   -->
   <Import Project="..\..\..\..\..\common\build.targets" />
-</Project>
+</Project>