WIP, utils: Implement the infrastructure for custom UI widget for effect configuration
WIP, EXAMPLE, customwidgets: Implement a custom widget for the alpha element
widget: Make NumericWidget usable as a wrapper around an existing Gtk.Range
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
Make the Alpha Filter's UI grid widget toplevel instead of inside a window
effects: Force the Alpha filter to always be in custom RGB chroma key mode
utils/widgets: Clean up the code and comments
utils/widgets: Fill sliders with color to indicate the default values

This provides a visual indication of which numeric values the user has changed.