Refactor all public static DockNotebook functions into a static class…

… called DockNotebookManager, and add some convenience methods for work we had in MainWindow.

Also update behavior:
- New tabs will be added to the active container rather than the first container.
- New tabs will be added to the right of the active tab rather than the end.

Pinta.Docking/Pinta.Docking.csproj

@@ -71,6 +71,7 @@
     <Compile Include="DockLibrary\TabStrip.cs" />
     <Compile Include="DockNotebook\DockNotebook.cs" />
     <Compile Include="DockNotebook\DockNotebookContainer.cs" />
+    <Compile Include="DockNotebook\DockNotebookManager.cs" />
     <Compile Include="DockNotebook\DockNotebookTab.cs" />
     <Compile Include="DockNotebook\DockWindow.cs" />
     <Compile Include="DockNotebook\PlaceholderWindow.cs" />