[FIXUP] "WIP, EXAMPLE, customwidgets: Implement a custom widget for the alpha element"
[FIXUP] utils/widgets: Hack to allow GESTrackElement children numeric props to work
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.