cmVisualStudio10TargetGenerator: Factor out helper for classic MSBuild project

In preparation for generating .Net SDK style project, refactor
cmVisualStudio10TargetGenerato::Generate to split the functionality to
write classic MSBuild project file. This commit only introduces a helper
function and moves the functionality there. A later commit will add
WriteSdkStyleProjectFile, the call to it, and the rest of the .Net
SDK-style changes.