Add Settings > Timeline > Adjustment Clip #1873

This pull request adds support for "Adjustment Clips" to the timeline, allowing users to add a special type of clip that applies effects to everything below it. The feature is only available when using libmlt version 7.41.0 or newer. The implementation includes UI changes to add the new generator, logic to create and insert the adjustment clip, and support for displaying it in the main window.

Adjustment Clip Feature:

• Added a menu option to insert an "Adjustment Clip" in the timeline when libmlt >= 7.41.0. (src/docks/timelinedock.cpp, src/docks/timelinedock.h) [[1]](diffhunk://#diff-f68df8db1fd177eef1313a1db87917a1f55cca11cd7d8474f227a25162845a18R1487-R1493) [[2]](diffhunk://#diff-ddb896addd53739957e7ec6971138cc519b45acb3cfb84ca42cbdda5192e2085R289-R291)


• Implemented the addAdjustmentClip() method to create and insert a color producer configured as an adjustment clip. (src/docks/timelinedock.cpp)


• Updated loadProducerWidget() to recognize adjustment clips and set up the filter controller appropriately. (src/mainwindow.cpp)