Move the Version member to the top cmLocalVisualStudioGenerator class
and set it consistently for instances created by all the global
generator versions.  Use an enumeration type with values scaled by a
factor of 10 so we can handle VS 7.1 without out-of-order numbers.

VS 7.1 support for SuppressStartupBanner was broken by commit 25116a3c
(Fix CMAKE_VERBOSE_MAKEFILE for VS10 vcxproj files, 2011-10-11) because
it assumed comparison of VS version numbers works.  Now it does.