This adds a PlatformToolset tag in order to make upgrading the projects to
Visual Studio 2012/2013 formats easier, especially when done with a script.
The script, for example, can change the value inside these tags
apporpriately to do the upgrade job quite easily.
This updates the Visual Studio 2010 Projects in the following ways,
similar to the recent changes to the Visual Studio 2008 projects:
-Make all the copying of the pre-configured header files custom build
rules, so that it is cleaner when people clean their builds, and the files
can be re-copied when updated.
-Split up the property sheets, so to ease future maintenance
-Make the cogl-path library built as a DLL
-Build and link against SDL-2.x for SDL builds
-Make everything except the .sln file and the README.txt file use UNIX line
endings, for easier maintenance.
-Merge cogl_sdl.sln and install-sdl.vcxproj into cogl.sln and
install.vcxproj respectively.
-Update build of the conformance test to not use COGL_COMPILATION, and make
it link to cogl-path.
