Improving Effect Widgets for Kdenlive

Kdenlive's effect system has three key UI gaps: the Curves effect lacks per-channel memory forcing users to apply it multiple times, the Gradient effect only supports two fixed color stops, and the Time Remapping panel has linear-only keyframe transitions with no easing. This project will build a tabbed per-channel Curves widget with backward-compatible serialization, a standalone GradientEditorWidget with arbitrary draggable color stops, and bezier handle support on RemapView connector lines with easing presets. All widgets integrate with AssetParameterModel and MLT's filter pipeline.