Update Addin to work with VS 2013 (preview)

For all developers which have to compile C++ projects, VS2012 is useless since it has a bug in the code optimizer and produces crashing code as soon as "toASCIILower()" is called. In contrast, in VS2013 preview this bug is fixed.

Therefore we have to expect many VS2012 devs to upgrade ASAP to VS2013 as soon as it is released - especially because the already released preview version has a go-live license.

We should anticipate this and prepare Qt and particularly the VS-addin as soon as possible to support VS2013.

Since VS2012 is similar to VS2012 in many details, I assume the adaption is primarily only a change in the installation path and in some install checks/procedures.