GtkRange API is incomplete

[Krzysztof Kosiński <tweenk pl gmail com>]

Hello

Here is my use use case (in Inkscape). I have a set of sliders
(GtkScale) that control some values in the document. I want to update
the view of the document in real time, but only push the change on the
undo stack when the drag is finished. But the only signal I can
connect to is value-changed. When I connect to it, I have only two
choices:
1. Update policy GTK_UPDATE_DISCONTINUOUS. This will give me correct
undo behavior but won't update the display in real time.
2. Update policy GTK_UPDATE_CONTINUOUS. This will update the display
correctly but will flood the undo stack with partial changes.

The problem would be solved if there was some additional signal that
would only be emitted when the user finishes the drag, or when the
slider is moved using the keyboard. It could be called value-set.
Essentially, it should be emitted after the value-changed signal
whenever the value-changed signal would be emitted if the update
policy was GTK_UPDATE_DISCONTINUOUS.

Here's the relevant bug report from our tracker.
https://bugs.launchpad.net/inkscape/+bug/579932

Regards, Krzysztof