Custom effects UI vulnerable to scroll

Steps to reproduce:

• Create a new project
• Add a video clip to the timeline
• Click the clip to select it
• Add the "3 point color balance" OR "alphaspot" effects to the clip
• Expand the effect item to reveal its properties UI
• Hover the R/G/B widgets and scroll with the mouse wheel, notice the widget value changes. This should not happen. See how we use the disable_scroll method in the codebase.

See how the UI is set up in:

• create_3point_color_balance_widget pitivi/utils/custom_effect_widgets.py#L185
• create_alphaspot_widget pitivi/utils/custom_effect_widgets.py#L350