Optional glowwindow and glowutils

Make the modules glowutils and glowwindow optional for the complete build process.
Therefore, two additional options (with default value ON) are introduced into the cmake configuration process:
 * OPTION_BUILD_GLOW_UTILS
 * OPTION_BUILD_GLOW_WINDOW

This has the following implications:
 * If either glowwindow or glowutils is deactivated, examples can not be build (a status message is showing what is missing if either one is deactivated but the option to build examples is set to true)
 * If glowwindow is deactivated, no glowwindow tests are build
 * If glowutils is deactivated, no glowutils tests are build