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/cmInstallExportGenerator.h

@@ -30,6 +30,7 @@ public:
                            const char* dest, const char* file_permissions,
                            const std::vector<std::string>& configurations,
                            const char* component,
+                           MessageLevel message,
                            const char* filename, const char* name_space,
                            bool exportOld, cmMakefile* mf);
   ~cmInstallExportGenerator();