Add standard user driver APIs for gentle scheduled update reminders

[zorgiepoo]

This allows developers to provide gentle reminders when scheduled update alerts would normally appear.

This is work a part of #924.

Documentation page written in sparkle-project/sparkle-project.github.io#138

Backgrounded apps no longer steal focus from other active applications when scheduled update alerts show up (unless the app just launched by the user). The update alert window is also not floating above all other app's windows anymore too. A logging message is done for backgrounded apps that schedule update checks but do not implement gentle reminder support.

The progress window (showing downloading, installing) frame position is also now in center to where the update alert window was last placed before being closed, and does not steal focus if the update alert wasn't key.

Foreground apps now wait until the app becomes re-activated before presenting a new update when opting into Sparkle handling showing the update.

APIs are now provided to allow developers to override Sparkle handling of showing updates, or add and dismiss gentle reminders both for foreground and background running applications.

Misc Checklist:

• My change requires a documentation update on Sparkle's website repository
• My change requires changes to generate_appcast, generate_keys, or sign_update

Only bug fixes to regressions or security fixes are being backported to the 1.x (master) branch now. If you believe your change is significant enough to backport, please also create a separate pull request against the master branch.

Testing

I tested and verified my change by using one or multiple of these methods:

• Sparkle Test App
• Unit Tests
• My own app
• Other (please specify)

Tested the samples in the documentation PR, while the app is a regular one and a background running one, app scheduled a check near launch and 10 seconds later, while the app was in-active and active (currently frontmost), and while the system has been idle vs has been in active use for non-background running app.

macOS version tested: 12.3 (21E230)

[Sparkle-Bot]

/Users/runner/work/Sparkle/Sparkle/Sparkle/SUUpdateAlert.m:91:5: warning: completion handler is never used when taking false branch [-Wcompletion-handler] if (self != nil) { ^ 1 warning generated. /Users/runner/work/Sparkle/Sparkle/Sparkle/SUUpdateAlert.m:91:5: warning: completion handler is never used when taking false branch [-Wcompletion-handler] if (self != nil) { ^ 1 warning generated. note: Using new build system note: Planning note: Build preparation complete note: Building targets in dependency order