Editors: Add 'is bidirectional' option to sliders

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.

source/blender/editors/include/ED_util.h

@@ -98,6 +98,9 @@ void ED_slider_factor_set(struct tSlider *slider, float factor);
 bool ED_slider_allow_overshoot_get(struct tSlider *slider);
 void ED_slider_allow_overshoot_set(struct tSlider *slider, bool value);
 
+bool ED_slider_is_bidirectional_get(struct tSlider *slider);
+void ED_slider_is_bidirectional_set(struct tSlider *slider, bool value);
+
 /* ************** XXX OLD CRUFT WARNING ************* */
 
 /**