Support Visual Studio 2010 and 2012

For Visual Studio 2010 and 2012, the scroll bar preview shows to the right of the scroll bar instead of within it. Starting with Visual Studio 2013, the scroll bar preview shows where it already did (in the middle of the scroll bar).

The target framework version was changed to .NET 4.0 to support Visual Studio 2010 without requiring users to install .NET 4.5+.

The project was also updated to support Round Trip loading of the project.

* Open the solution without warnings in any of Visual Studio 2010, 2012, or 2013
* Press F5 to debug in the experimental instance of the current version of Visual Studio (no need to manually configure debug settings)
It will probably be a separate pull request from the 2010-2012 support, as soon as I'm able to test it.