Update check fixes/improvements

[jdpurcell]

• aa03cf0 uncommented the flag to disable update check support, I'm assuming that was for testing and not intentionally committed.
• When fetching from the API, it requested the full release list. This caused unnecessary network traffic and would notify for pre-release versions. Now it asks just for the latest version.
• Changelog was displayed different on macOS vs Windows. This is because the changelog from the API uses \r\n as newline, whereas the splitting happened on \n, leaving \r still present. On macOS the notification dialog looked fine and had newlines, but this wasn't the case on Windows (see screenshots for before/after).
• Implement rate limiting for the automatic checks at startup, so it doesn't happen more frequently than every 4 hours.

[jurplel]

thanks! will get to more asap