Document the new methods for custom effects configuration UIs
utils/widgets: Compact code
[FIXUP]  widget: Make NumericWidget usable as a wrapper around an existing Gtk.Range
Use child properties for effects in dynamic widgets
[WIP?] utils/widgets: Hack to allow GESTrackElement children numeric props to work
utils/widgets: Update the value type when setting a new value
Apply various fixes for custom effect UIs
Add a custom user interface for chroma keying with the Alpha filter
Get the Alpha effect custom UI working