cmake-gui: Replace command-line install dialog with information box
The QMacInstallDialog infrastructure no longer works on modern OS X versions. Drop it and replace the dialog with a simple info box that explains to the user how to run 'cmake-gui --install' to add symlinks. Also suggest simply modifying the PATH. This approach is similar to the Xcode 'xcode-select --install' method of installing Xcode Command Line Tools.