Adds a verification on whether the process is trusted for Accessibility

[xrubioj]

This fixes/mitigates the issue #674: "Application will not restart after update in macOS Mojave".

There are two possible cases here:

• app is started for the first time (or permission was removed). In that case, permission for Radiant Player will be shown in the System Preferences -> Privacy -> Accessibility, but it will be unchecked. This is the most obvious case for the user.
• app has been updated. In this case, permission for Radiant Player will be shown in the System Preferences -> Privacy -> Accessibility and it will be checked. The user needs to uncheck it and then check it again. This is not obvious at all.

This fix shows an alert explaining both cases to the user, offers the user to open the System Preferences (should be shown in both cases, not only for the first case, which was the previous behavior). This makes clear that Radiant Player will close, and that the user needs to grant the permission and restart Radiant Player manually. Also, explains the steps to be taken for the second case, so it's clear for the user that even if the permission is checked, in needs to be disabled and reenabled.

[jacobwgillespie]

Thanks for this!

[jacobwgillespie]

After Travis CI builds, this should be released as v1.11.7.

[xrubioj]

Great! Thanks. Now I'm working in #675. I think I have a clue on what's going on. I'll post updates there. Hope to have it finished soon. 😄