Add a convenience function to check if the application is running
in macOS. Use this method to change the default visibility of on-
screen control buttons and function bar.
Neither of these are needed when running on macOS since both mouse
and keyboard are available. Also the mouse pointer in ScummVM does
not access the on-screen control buttons.
However this requires function keys and key combinations like e.g.
Alt+X to work to be able to save and quit games.
These will be added in future commits.
ceb54851c5