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/cmInstallFilesGenerator.h

@@ -28,6 +28,7 @@ public:
                           const char* file_permissions,
                           std::vector<std::string> const& configurations,
                           const char* component,
+                          MessageLevel message,
                           const char* rename,
                           bool optional = false);
   virtual ~cmInstallFilesGenerator();