Changes to display JVM installations on macOS/OSX.

Changes to display JVM installations on macOS/OSX.
Main change here is in Installer.java where scanDirectory was changed to work recursively, thanks to this we are able to find JVM on macOS which are in less predicable places ;-) on macOS JVM location will be for example /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home, but scaning /Library/Java/JavaVirtualMachines/ without this recursive thing will not find JVM.

Additionally changed text in MainWindow which suggests now that users should prefer alternate JVM over replace JVM, with short "explanation" why.