Inform end-users about why "System Events" access is being requested on macOS

On macOS, it's a fairly standard practice to display a pre-roll if your application needs some form of permissions, before calling the system APIs for the first time that will actually produce the system's requested-permissions window.

For instance, for any given end-user, the following window is likely A) intimidating, and B) possibly encourages insecure habits, such as simply clicking "accept" when they see a request like that:

Some examples of such pre-rolls from other macOS applications (mind you, some of these are for-pay examples; there's no need for ours to be fancy, just clear):

Good practices:

• Check if it's enabled before displaying pre-roll, just in case
• Only display pre-roll once; if it's explicitly been ignored/disabled by the user, then don't bother them
• Show exactly what functionality the user will lose if they ignore it / deny the permissions

Related: #753