install: Thread message level setting through internal API

Create a cmInstallGenerator::MessageLevel enumeration for future use in
specifying install message verbosity.  Thread values of the type through
constructors and save the value as a member of cmInstallGenerator.
Use only a "MessageDefault" value for now.

Source/cmInstallTargetGenerator.h

@@ -27,6 +27,7 @@ public:
     const char* file_permissions,
     std::vector<std::string> const& configurations,
     const char* component,
+    MessageLevel message,
     bool optional
     );
   virtual ~cmInstallTargetGenerator();