- Manage the brightness individually for each connected external monitor.
- Read the brightness from the target monitor if available; otherwise maintain a @[display_id: brightness] dictionary in the user defaults.
- Display the Brightness system OSD on the target monitor.
- Upgraded DDC to a more-recent version.
- Removed useless CoreGraphicsPriv.h.

If the app is not trusted in the Accessibility preference panel when started, it now waits that the user trusts it before installing the key event monitor. Previously the app had to be relaunched after being authorized to grab the keys, which was not user-friendly at all, because that the app is invisible and had to be killed via the Activity Monitor!!

- Waiting for accessibility authorization is now done using the selector-based NSTimer API, making it work on macOS versions < 10.12.
- Updated version information and copyright.

- Show the brightness level pane on the target display if the brightness level is not actually changed because already at the min or max level.
- [Internal] Declare C functions and variable as static in AppDelegate.m to better express their expected scope.

- Rewritten management of the brightness level, so that possible brightness levels are evenly distributed in the monitor's possible range.
- Added the support of quarter-step adjustments when the Option modifier key is pressed (similar to the system brightness management on the internal display when  Shift+Option is pressed).

- Brightness adjustment now targets the main screen if the `shift` key modifier is not pressed, and all screens if `shift` is pressed in conjunction with F1 or F2.
- Display the native brightness OSD on every adjusted screen.
- Support brightness adjustment of the builtin screen with F1 / F2 for consistency with external screens.