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
Use child properties for effects in dynamic widgets
utils/widgets: Allow introspected TrackElement props to work in NumericWidget
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.
utils/widgets: Allow introspected TrackElement props to work
in FractionWidget and ToggleWidget
utils/widgets: Make TextWidget with choices work with GTK3's ComboBox

This is used in some rare cases like the editable aspectratio combobox widget
for the "aspectratiocrop" effect.
Add debug statements for custom effects UIs and setting effect properties
effects: Remove an obsolete workaround for aspectratiocrop

With introspection, this workaround seems to do nothing now.
utils/widgets: Allow using a reset_all_button in custom effect UIs