Use VS2012 as the default for the builds.

We've been using VS2012 for the nightlies for some time now without any real issues apart from the renderer one which was reported and worked around.
This will allow us to use more C++11 features.

No files are renamed or removed at this point; we'll do this after 1.7.0 is out.
build.bat: tabs to spaces.
Enable LTCG for VS2012 by default for Release builds.

The reason we disabled LTCG in the past was issue https://trac.mpc-hc.org/ticket/2508 which I cannot reproduce with VS2012 Lite LTCG x64 builds so we can assume this is fixed now.