Add a field `is_bidirectional` to sliders (`tSlider`). By defafult this
is `false`; when set to `true` the slider allows negative values.
The allowed ranges are now:
- No overshoot, no bidirectional: `[0, 1]`
- No overshoot, bidirectional: `[-1, 1]`
- Overshoot, no bidirectional: `[0, infinity)`
- Overshoot, bidirectional: `(-infinity, infinity)`
This will be used for the pose library blending operator, where sliding
to the right (postitive) blends as normal, and sliding to the left
(negative) flips the pose before blending.
88748e5cbc