Fixed deadlock on macOS & added filtering of redundant layout requests

[janiselfert]

Summary

This PR fixes a critical deadlock issue in macOS printing that prevented the print dialog from working correctly in dynamic layout mode, prevents redundant onLayout calls and fixes problems with multiple windows.

Addressed issues:

1865
1875

Problem

The original implementation used DispatchSemaphore.wait() to block the main thread while waiting for the PDF document from Flutter. However, this caused a deadlock on newer Flutter versions which use merged threads (see here).

Solution

Replaced the semaphore-based blocking with RunLoop polling:

• Uses RunLoop.run(mode:before:) to keep the main thread responsive
• Allows Flutter Platform Channel to deliver the PDF document while "waiting"
• Includes a 30-second timeout as a safety measure
• The main thread processes events in 0.1s intervals until the document arrives

Additional Changes:

Layout Parameter Caching:

• macOS calls knowsPageRange() multiple times during the print dialog lifecycle with identical parameters
• Added caching of layout parameters to prevent redundant onLayout() calls

Shared PrintJobs between plugin instances:

• Each window creates a new PrintingPlugin instance, overwriting the singleton instance variable.
• Scenario: Jobs registered in Window 1 were stored in old instance, but setDocument() looked in new instance (Window 2) → job not found → 30s timeout
• Changed from instance-specific jobs dictionary to static sharedJobs list shared across all instances with NSLock to prevent errors caused by concurrent dictionary edits from different instances.
• Made jobID start at a random int, so overlaps between Plugin instances are unlikely
• This should not affect single instance scenarios, but fixes problems with multiple instances.